DEVELOPMENTAL ANATOMY (A) A A H (D) Aortic arches Truncus Diencephalon Choroid fissure FIGURE 1.6 Depictions of chick developmental anatomy. (A) Sensory retina下Eye Liver Dorsal view (looking"down"at what will become the back) of Pigmented retin 2-day chick embryo, as depicted by Marcello Malpighi in 1672 Olfactory pit (B)Ventral view (looking"up" at the prospective belly) of a chick Forelimb embryo at a similar stage, seen through a dissecting microscope and rendered by F R. Lillie in 1908. (C) Eduard d'Alton's depic- Somite Vitelline vein ion of a later stage 2-day chick embryo in Pander (1817).(Dy Modern rendering of a 3-day chick embryo. Details of the anato- Vitelline artery will be discussed in later chapters. (A from Malpighi 1672; Hindlimb from Lillie 1908: C from Pander 1817, courtesy of Emst Mayr Library of the Museum of Comparative Zoology, Harvard; D after Carlson 1981) cell theory arose in the mid-1800s), nor did they view mankind's tenure on Earth as potentially infinite. Rather, remain constant. Although certain microscopists claimed said Bonnet(1764),"Nature works as small as it wishes, see fully formed human miniatures within the sperm or and the human species existed in that finite time between egg, the major proponents of this hypothesis--Albrecht Creation and Resurrection. This view was in accord with von Haller and Charles Bonnet--knew that organ systems the best science of its time, conforming to the French math- develop at different rates, and that structures need not be ematician-philosopher Rene Descartes principle of the infi- in the same place in the embryo as they are in the newborn. nite divisibility of a mechanical nature initiated, but not The preformationists had no cell theory to provide a interfered with, by God. It also conformed to Enlighten lower limit to the size of their preformed organisms(the ment views of the Deity. The scientist-priest Nicolas Male-
DEVELOPMENTAL ANATOMY 13 (A) (B) % V-J (Q (D) Auditory vesicle Myelencephalon Heart < FIGURE 1.6 Depictions of chick developmental anatomy. (A) Dorsal view (looking "down" at what will become the back) of a 2-day chick embryo, as depicted by Marcello Malpighi in 1672. (B) Ventral view (looking "up" al the prospective belly) of a chick embryo at a similar stage, seen through a dissecting microscope and rendered by F. R. Lillie in 1908. (C) Eduard d'Alton's depiction of a later stage 2-day chick embryo in Pander (1817). (D) Modern rendering of a 3-day chick embryo. Details of the anatomy will be discussed in later chapters. (A from Malpighi 1672; B from Lillie 1908; C from Pander 1817, courtesy of Ernst Mayr Library of the Museum of Comparative Zoology, Harvard; D after Carlson 1981.) remain constant. Although certain microscopists claimed to see fully formed human miniatures within the sperm or egg, the major proponents of this hypothesis—Albrecht von HaJler and Charles Bonnet—knew that organ systems develop at different rates, and that structures need not be in the same place in the embryo as they are in the newborn. The preformationists had no cell theory to provide a lower limit to the size of their preformed organisms (the Aortic arches Atrium Truncus arterious Ventricle Liver rudiment Forelimb bud Somite Hindlimb bud Metencephalon Mesencephalon Diencephalon Choroid fissure Lens Sensory retina Pigmented retina Olfactory pit Telencephalon Vitelline vein Vitelline artery r r7e cell theory arose in the mid-1800s), nor did they view mankind's tenure on Earth as potentially infinite- Rather, said Bonnet (1764), "Nature works as small as it wishes," and the human species existed in that finite time between Creation and Resurrection. This view was in accord with the best science of its time, conforming to the French mathematician-philosopher Rene Descartes' principle of the infinite divisibility of a mechanical nature initiated, but not interfered with, by God. It also conformed to Enlightenment views of the Deity. The scientist-priest Nicolas Male-
14 CHAPTER 1 branche saw in preformationism the fusion of the rule-giv- instructions for forming the organism are already pres ing God of Christianity with Cartesian science( Churchill in the fertilized egg 1991; Pinto-Correia 1997). The embryological case for epigenesis was revived at Naming the parts: The primary germ layers the same time by Kaspar Friedrich Wolff, a German embry- and early organs ologist working in St. Petersburg. By carefully observing the development of chick embryos, Wolff demonstrated The end of preformationism did not come until the Is that the embryonic parts develop from tissues that have when a combination of new staining techniques, no counterpart in the adult organism. The heart and blood microscopes, and institutional reforms in German un vessels(which, according to preformationism, had to be sities created a revolution in descriptive embryology. T present from the beginning to ensure embryonic growth) new techniques enabled microscopists to document could be seen to develop anew in each embryo. Similarly, epigenesis of anatomical structures, and the institut the intestinal tube was seen to arise by the folding of an reforms provided audiences for these reports and stude originally flat tissue. This latter observation was explicitly to carry on the work of their teachers. Among the most ta detailed by Wolff, who proclaimed in 1767 that"When the ented of this new group of microscopically inclined in formation of the intestine in this manner has been duly tigators were three friends, born within a year of weighed, almost no doubt can remain, I believe, of the other, all of whom came from the Baltic region and st truth of epigenesis. To explain how an organism is creat- ied in northern Germany. The work of Christian Pa d anew each generation, however, Wolff had to postulate Karl Ernst von Baer, and Heinrich Rathke transfor an unknown force-the vis essentials ("essential force") embryology into a specialized branch of science. which, acting according to natural laws in the same way Pander studied the chick embryo for less than two ve as gravity or magnetism, would organize embryonic (before becoming a paleontologist), but in those 15 mon development e discovered the germ layers--three distinct region esis was attempted by the German philosopher Immanuel and specific organ systems(Figure 1. 7 tiated cells fyp A reconciliation between preformationism and epigen- the embryo that give rise to the differe Kant(1724-1804)and his colleague, biologist Johann Friedrich Blumenbach(1752-18 40). Attempting to construct The ectoderm generates the outer layer of the embo It produces the surface layer(epidermis) of the skin a scientific theory of racial descent, Blumenbach postulat forms the brain and nervous system. ed a mechanical, goal-directed force he called Bildungstrieb The endoderm becomes the innermost laver of th ("developmental force). Such a force, he said, was not the- embryo and produces the epithelium of the digest oretical, but could be shown to exist by experimentation Ahydra, when cut, regenerates its amputated parts by rear- tube and its associated organs (including the lungs. ranging existing elements(see Chapter 15). Some purpose The mesoderm becomes sandwiched between the ec ful organizing force could be observed in operation, and derm and endoderm. It generates the blood, heart, kd- this Bildungstrieb was a property of the organism itself, ney, gonads, bones, muscles, and connective tissues. thought to be inherited through the germ cells. Thus, These three layers are found in the embryos of a development could proceed through a predetermined force triploblastic(three-layer)animals. Some phyla, such as inherent in the matter of the embryo( Cassirer 1950: Lenoir the poriferans(sponges) and ctenophores(comb jellies 1980). Moreover, this force was believed to be susceptible to lack a true mesoderm and are considered diploblastic change, as demonstrated by the left-handed variant of snail animals coiling(where left-coiled snails can produce right-coiled Pander and Rathke also made observations that west. progeny). In this hypothesis, wherein epigenetic develop ed the balance in favor of epigenesis Rathke follow ment is directed by preformed instructions, we are not far intricate development of the vertebrate skull, excretory from the view held by modern biologists that most of the tems, and respiratory systems, showing that these be increasingly complex. He also showed that their comp "Preformation was a conservative theory, emphasizing the lack of ity took on different trajectories in different classes of ve change between generations. Its principal failure was its inability to tebrates. For instance, Rathke was the first to identifv th ount for the variations revealed by the limited genetic evidence and black parents produced children of intermediate skin color, an *But, as we shall see, not all the instructions there. Later nt impossibility if inheritance and development were solely through book, we will see that temperature, diet, predators, symbionts. either the sperm or the egg. In more controlled experiments, the crowding, and other environmental agents normally regulate German botanist Joseph Kolreuter(1766)produced hybrid tobacco expression in the embryo and can cause particular phenoxy ing the hybrid to either the male or female parent, Kolreuter was tFrom the same root as germination, the Latin germen means able to "revert"the hybrid back te one or the other parental type sprout"or"bud " The names of the three germ layers are fro after several generations. Thus, inheritance seemed to arise from a Greek: ectoderm from ektos ("outside")plus derma (skinE mixture of parental components derm from mesos("middle"); and endoderm from en
14 CHAPTER 1 branche saw in preformationism the fusion of the rule-giving God of Christianity with Cartesian science (Churchill 1991; Pinto-Correia 1997).* The embryological case for epigenesis wa s revived at the same time by Kaspar Friedrich Wolff, a German embryologist working in St. Petersburg. By carefully observing the development of chick embryos, Wolff demonstrated that the embryonic parts develop from tissues that have n o counterpart in the adult organism. The heart and blood vessels (which, according to preformationism, had to be present from the beginning to ensure embryonic growth) could be seen to develop anew in each embryo. Similarly, the intestinal tube wa s seen to arise by the folding of an originally flat tissue. This latter observation was explicitly detailed by Wolff, who proclaimed in 1767 that "When the formation of the intestine in this manne r has been duly weighed, almost no doubt can remain, I believe, of the truth of epigenesis." To explain how an organism is created anew each generation, however, Wolff had to postulate an unknown force—the vis essentialis ("essential force")— which, acting according to natural laws in the same way as gravity or magnetism, woul d organize embryonic development. A reconciliation between preformationism and epigenesis w a s attempted by the German philosopher Immanuel Kant (1724-1804) and his colleague, biologist Johann Friedrich Blumenbach (1752-1840). Attempting to construct a scientific theory of racial descent, Blumenbach postulated a mechanical, goal-directed force he called Bildungstrieb ("developmental force")- Such a force, he said, was not theoretical, but could be shown to exist by experimentation. A hydra, when cut, regenerates its amputated parts by rearranging existing elements (see Chapter 15). Some purposeful organizing force could be observed in operation, and this Bildungstrieb wa s a property of the organism itself, thought to be inherited through the germ cells. Thus, development could proceed through a predetermined force inherent in the matter of the embryo (Cassirer 1950; Lenoir 1980). Moreover, this force was believed to be susceptible to change, as demonstrated by the left-handed variant of snail coiling (where left-coiled snails can produce right-coiled progeny). In this hypothesis, wherein epigenetic development is directed by preformed instructions, we are not far from the view held by modern biologists that most of the ''Preformation was a conservative theory, emphasizing the lack of change between generations. Its principal failure was its inability to account for the variations revealed by the limited genetic evidence of the time. It was known, for instance, that matings between white and black parents produced children of intermediate skin color, an impossibility if inheritance and development were solely through either the sperm or the egg. In more controlled experiments, the German botanist Joseph Kolreutcr (1766) produced hybrid tobacco plants having the characteristics of both species. Moreover, by mating the hybrid to either the male or female parent, Kolrcuter was able to "revert" the hybrid back to one or the other parental type after several generations. Thus, inheritance seemed to arise from a mixture of parental components. instructions for forming the organism are already -.- --,. - in the fertilized egg.* Naming the parts: The primary germ layers and early organs The end of preformationism did not come until the 152/s when a combination of new staining techniques, impnn microscopes, and institutional reforms in German uni sities created a revolution in descriptive embryology, new techniques enabled microscopists to document epigenesis of anatomical structures, and the institutkr reforms provided audiences for these reports and stiiders to carry on the work of their teachers. Among the most taK ented of this new group of microscopically inclined n i tigators were three friends, born within a year of eac other, all of whom came from the Baltic region and staw ied in northern Germany. The work of Christian Panda Karl Ernst von Baer, and Heinrich Rathke transfons embryology into a specialized branch of science. Pander studied the chick embryo for less than tw: (before becoming a paleontologist), but in those 15 mor_i~ he discovered the germ layers+ —three distinct regions if the embryo that give rise to the differentiated celL and specific organ systems (Figure 1.7). • The ectoderm generates the outer layer of the embrrt It produces the surface layer (epidermis) of the skin a forms the brain and nervous system. • The endoderm becomes the innermost layer of ti embryo and produces the epithelium of the digest* tube and its associated organs (including the lungs). • The mesoderm becomes sandwiched between the ed o derm and endoderm. It generates the blood, heart, kiiney, gonads, bones, muscles, and connective tissues. These three layers are found in the embrvos of t triploblastic ("three-layer") animals. Some phyla, such, the poriferans (sponges) and ctenophores (comb jeffies) lack a true mesoderm and are considered diploblastir animals. Pander and Rathke also made observations that weigh* ed the balance in favor of epigenesis. Rathke follow e i I intricate development of the vertebrate skull, excr;:;: terns, and respiratory systems, showing that these betas increasingly complex. He also showed that their compie ity took on different trajectories in different classes of TI tebrates. For instance, Rathke was the first to idenfcr *But, as we shall see, not all the instructions there. Late in iias book, we will see that temperature, diet, predators, symbionE, crowding, and other environmental agents normally regulate expression in the embryo and can cause particular phenoc-T -; occur. + From the same root as germination, the Latin germen mearts "sprout" or "bud." The names of the three germ layers a t: Greek: ectoderm from ektos ("outside") plus derma ("skin"k derm from mesos ("middle"); and endoderm from endtm
DEVELOPMENTAL ANATOMY 15 Gastrula Mesoderm(middle layer) Endoderm(internal layer) Germ cells Central Outer nervous Neural Digestive 画米色a③ cell chord tissue cell of the blood muscle brain(melan alveolar FIGURE 1. The dividing cells of the fertilized egg form three distinct embryonic germ layers. Each of the germ layers gives rise pharyngeal arches(Figure 1. 8). He showed that these same to myriad differentiated cell types(only a few representatives are embryonic structures became gill supports in fish and the shown here) and distinct organ systems. The germ cells(precur- jaws and ears(among other things) in mammals. Pander sors of the sperm and egg)are de early in development and demonstrated that the germ layers did not form their do not arise from any particular germ layer respective organs autonomously(Pander 1817). Rather, each germ layer"is not yet independent enough to indi- cate what it truly is; it still needs the help of its sister trav elers, and therefore, although already designated for dif ferent ends, all three influence each other collectively until each has reached an appropriate level. " Pander had dis arch structures in the vertebrate head. (A) Lower jaw Hyomandibular Pharyngeal arches (also called branchial arches) in the embryo of the salamander Ambystoma mexicanum. The surface ecto- derm has been removed to permit visuali (C) zation of the arches (highlighted in color) one they form. (B)In adult fish, pharr Nasal Maxilla Quadrate arch cells form the hyomandibular jaws d gill arches. (C)In amphibians, birds Nasal d reptiles(a crocodile is shown here) Premaxilla hese same cells form the quadrate bone of the upper jaw and the articular bone of the lower jaw. (D)In mammals, the qua- NA-ATURAUAAVEA drate has become internalized and forms the incus of the middle ear. The articular Middle ear bone retains its contact with the quadrate, Incus, becoming the malleus of the middle ear. Articular (A courtesy of P. Falck and L Olsson; B-D Maxilla Mandible after Zangerl and Williams 1975.)
DEVELOPMENTAL ANATOMY 15 Zygote Blastula Gastrula r Ectoderm (outer layer) Mesoderm (middle layer) Central Outer nervous Neural J. Endoderm (internal layer) Germ cells Digestive Respiratory surface system crest Dorsal Paraxial Intermediate Lateral Head tube Pharynx tube 0gk Male Female Epidermal Neuron cells of skin of brain Pigment cell (melanocyte) Notochord Bone tissue Tubule cell of the kidney Red blood cells Facial muscle Stomach cell Thvroid cell Lung cell (alveolar cell) FIGURE 1.7 The dividing cells of the fertilized egg form three distinct embryonic germ layers. Each of the germ layers gives rise to myriad differentiated cell types (only a few representatives are shown here) and distinct organ systems. The germ cells (precursors of the sperm and egg) are set aside early in development and do not arise from any particular germ layer. pharyngeal arches (Figure 1.8). He showed that these same embryonic structures became gill supports in fish and the jaws and ears (among other things) in mammals. Pander demonstrated that the germ layers did not form their respective organs autonomously (Pander 1817). Rather, each germ layer "is not yet independent enough to indicate what it truly is; it still needs the help of its sister travelers, and therefore, although already designated for different ends, all three influence each other collectively until each has reached an appropriate level." Pander had dis- (B) Upper jaw Braincase Gill arches (C) Nasal Maxilla Squamosal Quadrate (D) Premaxilla Hvomandibula Squamosal (temporal bone) Nasal Dentary Articular Maxilla Middle ear (incus, malleus) Mandible FIGURE 1.8 Evolution of pharyngeal arch structures in the vertebrate head. (A) Pharyngeal arches (also called branchial arches) in the embryo of the salamander Ambystoma mexicanum. The surface ectoderm has been removed to permit visualization of the arches (highlighted in color) as they form. (B) In adult fish, pharyngeal arch cells form the hyomandibular jaws and gill arches. (C) In amphibians, birds, and reptiles (a crocodile is shown here), these same cells form the quadrate bone of the upper jaw and the articular bone of the lower jaw. (D) In mammals, the quadrate has become internalized and forms the incus of the middle car. The articular bone retains its contact with the quadrate, becoming the malleus of the middle ear. (A courtesy of P. Falck and L. Olsson; B-D after Zangerl and Williams 1975.)
16 CHAPTER 1 Ectoderm of head and left halves and which instructs the ectoderm above it to become the nervous system(Figure 1.9). He also discov Area pellucida ered the mammalian egg, that long-sought cell that every one believed existed but no one before von Baer had ever seen. In 1828, von Baer reported, I have two small embryos Notochord preserved in alcohol, that I forgot to label. At present I am unable to determine the genus to which they belong. They may be lizards, small birds, or even mammals. " Figure 1.10 allows us to appreciate his quandary. All vertebrate embryos(fish, reptiles, amphibians, birds, and mammals) with a basically similar structure. From his de study of chick development and his comparison of chick embryos with the embryos of other vertebrates, von Baer Hensen's derived four generalizations. Now often referred to as"von Baers laws, "they are stated here with some vertebrate examples streak 1. The general features of a large group of animals appear earlier in development than do the specialized features of a Somite(source of Neural groove smaller group. All developing vertebrates appear very muscles, spine, ribs similar right after gastrulation. It is only later in devel Epidermal ectoderm opment that the special features of class, order, and von Baer could hardly believe that he had at last found what so many others--Harvey, de Graaf, von Haller, Prevost, Dumas, and even Purkinje-had searched for and failed to find. "I recoiled as if struck by lightening.. I had to try to relax a while before I could work up enough courage to look again, as I was afraid I had been Lateral plate Hiate Notochord Endoderm deluded by a phantom. Is it not strange that a sight which is expect and indeed hoped for, should be frightening when it eventually (source of of materializes? heart, blood kidneys, gonads FIGURE 1.9 Notochord in chick development. The notochord separates vertebrate embryos into right and left halves and instructs the ectoderm above it to become the nervous system A)Dorsal view of the 24-hour chick embryo. ( B) Cross section through the trunk region shows the notochord and developing neural tube. By comparing this illustration and Figure 1.6,you can see the remarkable changes between days 1, 2, and 3 of hick egg incubation. (A after Patten 1951) overed the tissue interactions that we now call induction No tissue is able to construct organs by itself; it must inter act with other tissues. (We will discuss the principles of induction more thoroughly in Chapter 3. )Thus, Pander showed that preformation could not be true, since the organs come into being through interactions between sim- Human Opossum Chicken Salamander Fish pler structures FIGURE 1.10 Similarities and differences among vertebrate The four principles of Karl Ernst von Baer embryos as they proceed through development. Each species" embryos begin with a basically similar structure, although they Karl Ernst von Baer extended Pander's studies of the chick acquire this structure at different ages and sizes. As they develop, embryo. He discovered the notochord, the rod of dor- the species become less like each other. (Adapted from Richard salmost mesoderm that separates the embryo into right son et al. 1998; photograph courtesy of M. Richardson
16 CHAPTER 1 Ectoderm of head Area pellucida Unsegmented mesoderm Hensen's node Primitive streak Somite (source of Neural groove muscles, spine, ribs) Epidermal ectoderm Lateral plate" mesoderm (source of heart, blood vessels) Intermediate mesoderm (source of kidneys, gonads) Notochord F.ndoderm (gut, lungs) FIGURE 1.9 Notochord in chick development. The notochord separates vertebrate embryos into right and left halves and instructs the ectoderm above it to become the nervous system. (A) Dorsal view of the 24-hour chick embryo. (B) Cross section through the trunk region shows the notochord and developing neural tube. By comparing this illustration and Figure 1.6, you can see the remarkable changes between days 1, 2, and 3 of chick egg incubation. (A after Patten 1951.) covered the tissue interactions that we now call induction. No tissue is able to construct organs by itself; it must interact with other tissues. (We will discuss the principles of induction more thoroughly in Chapter 3.) Thus, Pander showed that preformation could not be true, since the organs come into being through interactions between simpler structures. The four principles of Karl Ernst von Baer Karl Ernst von Baer extended Pander's studies of the chick embryo. He discovered the notochord, the rod of dorsalmost mesoderm that separates the embryo into right and left halves and which instructs the ectoderm above it to become the nervous system (Figure 1.9). He also discovered the mammalian egg, that long-sought cell that everyone believed existed but no one before von Baer had ever seen.* In 1828, von FJaer reported, "I have two small embrj'os preserved in alcohol, that I forgot to label. At present I am unable to determine the genus to which they belong. They may be lizards, small birds, or even mammals." Figure 1.10 allows us to appreciate his quandary. All vertebrate embryos (fish, reptiles, amphibians, birds, and mammals) begin with a basically similar structure. From his detailed study of chick development and his comparison of chick embryos with the embryos of other vertebrates, von Baer derived four generalizations. Now often referred to as "von Baer's laws," they are stated here with some vertebrate examples. 1. The general features of a large group of animals appear earlier in development than do the specialized features of a smaller group. All developing vertebrates appear very similar right after gastrulation. It is only later in development that the special features of class, order, and *von Baer could hardly believe that he had at last found what so many others—Harvey, de Graaf, von Haller, Prevost, Dumas, and even Purkinje—had searched for and failed to find. "I recoiled as if struck by lightening ... I had to try to relax a while before I could work up enough courage to look again, as I was afraid I had been deluded by a phantom. Is it not strange that a sight which is expect- ed, and indeed hoped for, should be frightening when it eventually materializes?" Human Opossum Chicken Salamander Fish (axolotl) (gar) FIGURE 1.10 Similarities and differences among vertebrate embryos as they proceed through development. Each species' embryos begin with a basically similar structure, although they acquire this structure at different ages and sizes. As they develop, the species become less like each other. (Adapted from Richardson ct al. 1998; photograph courtesy of M. Richardson.)
DEVELOPMENTAL ANATOMY 17 finally species emerge. All vertebrate embryos have arches, a notochord, a spinal cord, and primitive kid Keeping Track of Moving Cells Fate Maps and Cell Lineages 2 Less general characters develop from the more general, until By the late 1800s, the cell had been conclusively demon- fmally the most specialized appear. All vertebrates initial- strated to be the basis for anatomy and physiology. Embry ly have the same type of skin. Only later does the skin ologists, too, began to base their field on the cell. But unlike hose who studied the adult, developmental anatomist hair, claws, and nails of mammals. Similarly, the early found that cells do not stay still in the embryo. Indeed, one of development of limbs is essentially the same in all ver- the most important conclusions of developmental tebrates. Only later do the differences between legs, anatomists is that embryonic cells do not remain in one wings, and arms become apparent place, nor do they keep the same shape(Larsen and The embryo of a given species, instead of passing through the McLaughlin 1987) Early embryologists recognized that there are two major them. The visceral clefts of embryonic birds and mam types of cells in the embryo: epithelial cells, which are mals do not resemble the gill slits of adult fish in detail. tightly connected to one another in sheets or tubes; and Rather, they resemble the visceral clefts of embryonic fish mesenchymal cells, which are unconnected to one anoth- and other embryonic vertebrates. Whereas fish preserve er and operate as independent units. Morphogenesis is and elaborate these clefts into true gill slits, mammals brought about through a limited repertoire of variations in convert them into structures such as the eustachian ellular processes within these two types of arrangements tubes (between the ear and mouth (Table 1. 1) Therefore, the early embryo of a higher animal is never like a Direction and number of cell divisions. Think of the faces of lower animal, but only like its early embryo. Human two dog breeds-say, a German shepherd and a poodle embryos never pass through a stage equivalent to an The faces are made from the same cell types, but the adult fish or bird. Rather, human embryos initially share number and orientation of the cell divisions are differ characteristics in common with fish and avian embryos ent. Think also of the legs of a German shepherd com- of them passing through the stages of the othe e, none pared with those of a dachshund. The skeleton-forming Later, the mammalian and other embryos diver sions than those of taller dogs(see Figure 1.21) to all vertebrate development: each of the three germ lay-. Cell shape changes. Cell shape change is a critical part of ers generally gives rise to the same organs, whether the ganism itself is a fish, a frog, or a chick ment,change in the shapes of epithelial cells often cre- Comparative embryonic anatomy remains an active ates tubes out of sheets(as when the neural tube forms) feld of research today, although it is now done in an eve and a shape change from epithelial to mesenchymal is onary context. What embryonic interactions, for critical when cells migrate away from the epithelium(as nstance, cover the squirrel's tail with fur but provide scales when muscle cells are formed). This same type of epithe- n the rats tail? The author s own research concerns he lial-to-mesenchymal change allows cancer cells to artes get their shells--a skeletal feature generally com migrate and spread from the primary tumor to new sites posed of 59 bones that no other vertebrate possesses. What Cell movement. Cell migration is critical to get cells to their appropriate places. The germ cells have to migrate into alligators and prehistoric marine reptiles? What changes the developing gonad, and the primordial heart cells in the"typical"development of the vertebrate skeleton meet in the middle of the vertebrate neck and then allowed these unique bones to form? Jack Horner and Hans Larsson are looking at the similarities between the evelopmental anatomy of chick and dinosaur embryos ent in the germ cells: the sperm eliminates most of its and have found that the embryonic chick, unlike the cytoplasm and becomes smaller, whereas the develop- regresses its tail. They are conducting experi ing egg conserves and adds cytoplasm, becoming com- ents to block this regression, and actually hope to obtain paratively huge. Many cells undergo an"asymmetric a chick that more closely resembles its dinosaur ancestors cell division that produces one big cell and one small Horner and Gorman 2009) cell, each of which may have a completely different fate Cell death. Death is a critical part of life. The cells that in the womb constitute the webbing between our toes and fingers die before we are born. So do the cells of our tails Baer formulated these generalizations prior to Darwins theo- The orifices of our mouth, anus, and reproductive glands of evolution. "Lower animals we ll form through cells dying at particular times and places
DEVELOPMENTAL ANATOMY 17 finally species emerge. All vertebrate embryos have gill arches, a notochord, a spinal cord, and primitive kidneys. 2. Less general characters develop from the more general, until finally the most specialized appear. All vertebrates initialhave the same type of skin. Only later does the skin A: velop fish scales, reptilian scales, bird feathers, or the ir, claws, and nails of mammals. Similarly, the early development of limbs is essentially the same in all vertebrates. Only later do the differences between legs, wings, and arms become apparent. 3. The embryo of a given specks, instead of passing through the adult stages of lower animals, departs more and more from diem* The visceral clefts of embryonic birds and mammals do not resemble the gill slits of adult fish in detail. Rather, they resemble the visceral clefts of embryonic fish and other embryonic vertebrates. Whereas fish preserve and elaborate these clefts into true gill slits, mammals convert them into structures such as the eustachian rubes (between the ear and mouth). 4. Therefore, the early embryo of a higher animal is never like a lower animal, but only like its early embryo. Human embryos never pass through a stage equivalent to an adult fish or bird. Rather, human embryos initially share characteristics in common with fish and avian embryos. Later, the mammalian and other embryos diverge, none of them passing through the stages of the others. nxi Baer also recognized that there is a common pattern to all vertebrate development: each of the three germ lays ;:;nerally gives rise to the same organs, whether the rrganism itself is a fish, a frog, or a chick. Comparative embryonic anatomy remains an active field of research today, although it is now done in an evo- :: nary context. What embryonic interactions, for sistance, cover the squirrel's tail with fur but provide scales r. the rat's tail? The author's own research concerns how -uriles get their shells—a skeletal feature generally comrvsed of 59 bones that no other vertebrate possesses. What I: • c relationship of these 59 bones to the bones found in Iligators and prehistoric marine reptiles? What changes • :he "typical" development of the vertebrate skeleton allowed these unique bones to form? Jack Horner and Hans Larsson are looking at the similarities between the developmental anatomy of chick and dinosaur embryos and have found that the embryonic chick, unlike the dinosaur, regresses its tail. They are conducting experiments to block this regression, and actually hope to obtain a chick that more closely resembles its dinosaur ancestors Homer and Gorman 2009). von Baer formulated these generalizations prior to Darwin's theo- -' evolution. "Lower animals" would be those having simpler sr-atotnies. Keeping Track of Moving Cells: Fate Maps and Cell Lineages By the late 1800s, the cell had been conclusively demonstrated to be the basis for anatomy and physiology. Embryologists, too, began to base their field on the cell. But unlike those who studied the adult, developmental anatomist found that cells do not stay still in the embryo. Indeed, one of the most important conclusions of developmental anatomists is that embryonic cells do not remain in one place, nor do they keep the same shape (Larsen and McLaughlin 1987). Early embryologists recognized that there are two major types of cells in the embryo: epithelial cells, which are tightly connected to one another in sheets or tubes; and mesenchymal cells, which are unconnected to one another and operate as independent units. Morphogenesis is brought about through a limited repertoire of variations in cellular processes within these two types of arrangements (Table 1.1): • Direction and number of cell divisions. Think of the faces of two dog breeds—say, a German shepherd and a poodle. The faces are made from the same cell types, but the number and orientation of the cell divisions are different. Think also of the legs of a German shepherd compared with those of a dachshund. The skeleton-forming cells of the dachshund have undergone fewer cell divisions than those of taller dogs (see Figure 1.21). • Cell shape changes. Cell shape change is a critical part of not only of development but also of cancer. In development, change in the shapes of epithelial cells often creates tubes out of sheets (as when the neural tube forms); and a shape change from epithelial to mesenchymal is critical when cells migrate away from the epithelium (as when muscle cells are formed). This same type of epithelial-to-mesenchymal change allows cancer cells to migrate and spread from the primary tumor to new sites. • Cell movement. Cell migration is critical to get cells to their appropriate places. The germ cells have to migrate into the developing gonad, and the primordial heart cells meet in the middle of the vertebrate neck and then migrate to the left part of the chest. • Cell growth. Cells can change in size. This is most apparent in the germ cells: the sperm eliminates most of its cytoplasm and becomes smaller, whereas the developing egg conserves and adds cytoplasm, becoming comparatively huge. Many cells undergo an "asymmetric" cell division that produces one big cell and one small cell, each of which may have a completely different fate. • Cell death. Death is a critical part of life. The cells that in the womb constitute the webbing between our toes and fingers die before we are bom. So do the cells of our tails. The orifices of our mouth, anus, and reproductive glands all form through cells dying at particular times and places