from one generation to the next. How are these germ cells set apart from the cells that are constructing the physical structures of the embryo, and what are the instructions in the nucleus and cytoplasm that allow them to form the next generation? The question of regeneration. Some organisms can regenerate their entire body. Some salamanders regenerate their eyes and legs, and many reptiles can regenerate their tails. Mammals are generally poor at regeneration, and yet there are some cells in our bodies-stem cells--that are able to form new structures even in adults. How do the stem cells retain this ca and can we harness it to cure debilitating diseases? The question of evolution. Evolution involves inherited changes in devel opment. When we say that today s one-toed horse had a five-toed ancestor, e are saying that changes in the development of cartilage and muscles occurred over many generations in the embryos of the horses ancestors How do changes in development create new body forms? Which heritable changes are possible, given the constraints imposed by the necessity that the organism survive as it develops? The question of environmental integration. The development of many (perhaps all) organisms is influenced by cues from the environment that sur- rounds the embryo or larvae. The sex of many species of turtles, for instance, depends on the temperature the embryo experiences while in the egg. The formation of the reproductive system in some insects depends on bacteria that are transmitted inside the egg. Moreover, certain chemicals in the environment can disrupt normal development, causing malformations in the adult. How is the development of an organism integrated into the larger context of its habitata The study of development has become essential for understanding all other areas of biology. Indeed, the questions asked by developmental biologists have also become critical in molecular biology, physiology, cell biology, genetics, anatomy, cancer research, neurobiology, immunology, ecology, and evolutionary biology. In turn, the many advances of molecular biology, along with new techniques of cell imaging, have finally made these questions answerable. This makes developmental biologists extremely happy; for, as the Nobel Prize-winning developmental biologist Hans Spe- mann stated in 1927 We stand in the presence of riddles, but not without the hope of solving them And riddles with the hope of solution-what more can a scientist desire? So, like the man in the cartoon, I come bearing questions. They are questions bequeathed to us by earlier generations of biologists, philosophers, and parents. They are questions with their own history, questions discussed on an anatomical level by people such as Aristotle, William Harvey, St. Albertus Magnus, and Charles Darwin More recently, these questions have been addressed on the cellular and molecular levels by men and women throughout the world, each of whom brings to the labo ratory his or her own perspectives and training. For there is no one way to become a developmental biologist, and the field has benefitted by having researchers trained in cell biology, genetics, biochemistry, immunology, and even anthropology, engineer g, physics, history, and art. You are now invited to become part of a community of question-askers for whom the embryo is a source of both wonder and the most inter ng questions In the wor The next three chapters outline some of the critical framework needed to answer these questions. Chapter 1 discusses organismal concepts, including life cycles, the
*-< W1_ J I IUI1 J from one generation to the next. How are these germ cells set apart from the cells that are constructing the physical structures of the embryo, and what are the instructions in the nucleus and cytoplasm that allow them to form the next generation? • The question of regeneration. Some organisms can regenerate their entire body. Some salamanders regenerate their eyes and legs, and many reptiles can regenerate their tails. Mammals are generally poor at regeneration, and yet there are some cells in our bodies—stem cells—that are able to form new structures even in adults. How do the stem cells retain this capacity, and can we harness it to cure debilitating diseases? • The question of evolution. Evolution involves inherited changes in development. When we say that today's one-toed horse had a five-toed ancestor, we are saying that changes in the development of cartilage and muscles occurred over many generations in the embryos of the horse's ancestors. How do changes in development create new body forms? Which heritable changes are possible, given the constraints imposed by the necessity that the organism survive as it develops? • The question of environmental integration. The development of many (perhaps all) organisms is influenced by cues from the environment that surrounds the embryo or larvae. The sex of many species of turtles, for instance, depends on the temperature the embryo experiences while in the egg. The formation of the reproductive system in some insects depends on bacteria that are transmitted inside the egg. Moreover, certain chemicals in the environment can disrupt normal development, causing malformations in the adult. How is the development of an organism integrated into the larger context of its habitat? The study of development has become essential for understanding all other areas of biology. Indeed, the questions asked by developmental biologists have also become critical in molecular biology, physiology, cell biology, genetics, anatomy, cancer research, neurobiology, immunology, ecology, and evolutionary biology. In turn, the many advances of molecular biology, along with new techniques of cell imaging, have finally made these questions answerable. This makes developmental biologists extremely happy; for, as the Nobel Prize-winning developmental biologist Hans Spemann staled in 1927, We stand in the presence of riddles, but not without the hope of solving them. And riddles with the hope of solution—what more can a scientist desire? So, like the man in the cartoon, I come bearing questions. They are questions bequeathed to us by earlier generations of biologists, philosophers, and parents. They are questions with their own history, questions discussed on an anatomical level by people such as Aristotle, William Harvey, St. Albertus Magnus, and Charles Darwin. More recently, these questions have been addressed on the cellular and molecular levels by men and women throughout the world, each of whom brings to the laboratory his or her own perspectives and training. For there is no one way to become a developmental biologist, and the field has benefitted by having researchers trained in cell biology, genetics, biochemistry, immunology, and even anthropology, engineering, physics, history, and art. You are now invited to become part of a community of question-askers for whom the embryo is a source of both wonder and the most interesting questions in the world. The next three chapters outline some of the critical framework needed to answer these questions. Chapter 1 discusses organismal concepts, including life cycles, the
4 PART I three germ layers that form the organs, and the migration of cells during develop- ment. Chapter 2 concentrates on the genetic approach to cell differentiation and out- lines the principle of differential gene expression (which explains how different pro- teins can be made in different cells from the same set of inherited genes). Chapter 3 focuses on the cellular approach to morphogenesis, showing how communication between cells is critical for their formation into tissues and organs. Thus, you will be introduced to development at the organismal, genetic, and cellular, and much of the textbook thereafter will show how these levels are integrated to produce the remark- able panoply of animal development
PART three germ layers that form the organs, and the migration of cells during development. Chapter 2 concentrates on the genetic approach to cell differentiation and outlines the principle of differential gene expression (which explains how different proteins can be made in different cells from the same sel of inherited genes). Chapter 3 focuses on the cellular approach to morphogenesis, showing how communication between cells is critical for their formation into tissues and organs. Thus, you will be introduced to development at the organismal, genetic, and cellular, and much of the textbook thereafter will show how these levels are integrated to produce the remarkable panoply of animal development
Developmental Anatomy ACCORDING TO ARISTOTLE, the first embryologist known to history, science It is a most beautiful thing to stud begins with wonder: "It is owing to wonder that people began to philosophize, the different changes of life, from the and wonder remains the beginning of knowledge"(Aristotle, Metaphysics, ca microscopic changes of conception to 350 BCE). The development of an animal from an egg has been a source of won- the more apparent ones of maturity each successive day of its 3-week incubation period provides a remarkable expe- and old age rience as a thin band of cells is seen to give rise to an entire bird. Aristotle per- FRANKLIN MALL (CA 1890) formed this procedure and noted the formation of the major organs. Anyone can wonder at this remarkable -yet commonplace--phenomenon, but it is the sci- The greatest progressive minds of entist seeks to discover how development actually occurs And rather than dis embryology have not looked for sipating wonder, new understanding increases it. hypotheses: they have looked at Multicellular organisms do not spring forth fully formed. Rather, they arise embryos by a relatively slow process of progressive change that we call development. In IANE OPPENHEIMER(1955) nearly all cases, the development of a multicellular organism begins with a sin- gle cell-the fertilized egg, or zygote, which divides mitotically to produce all the cells of the body. The study of animal development has traditionally been called embryology, after that phase of an organism that exists between fertiliza- tion and birth But development does not stop at birth, or even at adulthood. Most organisms never stop develop of skin cells(the older cells being sloughed off as we move), and our bone mar- row sustains the development of millions of new red blood cells every minute of our lives. In addition, some animals can regenerate severed parts, and many species undergo metamorphosis(such as the transformation of a tadpole into a frog, or a caterpillar into a butterfly). Therefore, in recent years it has become customary to speak of developmental biology as the discipline that studies embryonic and other developmental processes As the introduction to Part I notes, a scientific field is defined by the q tions it seeks to answer. Most of the questions in developmental biology have been provided to it by its embryological heritage. We can identify three major approaches to studying embryology Anatomical approache Experimental approaches Genetic approaches Each of these traditions has predominated during a different era. However, although it is true that anatomical approaches gave rise to experimental approach- es, and that genetic approaches built on the foundations of the earlier two approaches, all three traditions persist to this day and continue to play a ma gy is the changing anatomy of the organism. Today the anatomical approach to
Developmental Anatomy ACCORDING TO ARISTOTLE, the first embryologist known to history, science begins with wonder: "It is owing to wonder that people began to philosophize, and wonder remains the beginning of knowledge" (Aristotle, Metaphysics, ca. 350 BCE). The development of an animal from an egg has been a source of wonder throughout history. The simple procedure of cracking open a chick egg on each successive day of its 3-week incubation period provides a remarkable experience as a thin band of cells is seen to give rise to an entire bird. Aristotle perrormed this procedure and noted the formation of the major organs. Anyone can wonder at this remarkable—yet commonplace—phenomenon, but it is the scientist seeks to discover how development actually occurs. And rather than dissipating wonder, new understanding increases it. Multicellular organisms do not spring forth fully formed. Rather, they arise by a relatively slow process of progressive change that we call development. In nearly all cases, the development of a multicellular organism begins with a single cell—the fertilized egg, or zygote, which divides mitotically to produce all the cells of the body. The study of animal development has traditionally been called embryology, after that phase of an organism that exists between fertilization and birth. But development does not stop at birth, or even at adulthood. Most organisms never stop developing. Each day we replace more than a gram of skin cells (the older cells being sloughed off as we move), and our bone marrow sustains the development of millions of new red blood cells every minute of our lives. In addition, some animals can regenerate severed parts, and many species undergo metamorphosis (such as the transformation of a tadpole into a frog, or a caterpillar into a butterfly). Therefore, in recent years it has become customary to speak of developmental biology as the discipline that studies embryonic and other developmental processes. As the introduction to Part I notes, a scientific field is defined by the questions it seeks to answer. Most of the questions in developmental biology have been provided to it by its embryological heritage. We can identify three major approaches to studying embryology: • Anatomical approaches • Experimental approaches • Genetic approaches Each of these traditions has predominated during a different era. However, although it is true that anatomical approaches gave rise to experimental approaches, and that genetic approaches built on the foundations of the earlier two approaches, all three traditions persist to this day and continue to play a major role in developmental biology. The basis of all research in developmental biology is the changing anatomy of the organism. Today the anatomical approach to It is a most beautiful thing to study the different changes of life, from the microscopic changes of conception to the more apparent ones of maturity and old age. FRANKLIN MALL (CA. 1890) The greatest progressive minds of embiyology have not looked for hypotheses; they have looked at embryos. JANE OPPENHEIMER (1955 )
6 CHAPTER 1 development is continually expanded and enhanced by rev- 5. In many species, the organism that hatches from the egg olutions in microscopy, computer-aided graphical recon- or is born into the world is not sexually mature. Rather structions of three-dimensional objects, and methods of the organism needs to undergo metamorphosis to applying mathematics to biology. Many of the beautiful become a sexually mature adult. In most animals, the photographs in this book reflect this increasingly impor- oung organism is a called a larva, and it may look sig tant component of embryology nificantly different from the adult. In many species, the larval stage is the one that lasts the longest, and is used The Cycle of Life for feeding or dispersal. In such species, the adult is a brief stage whose sole purpose is to reproduce. In silk One of the major triumphs of descriptive embryology was worm moths, for instance, the adults do not have the idea of a generalizable animal life cycle. Each animal, mouthparts and carnot feed; the larvae must eat whether earthworm or eagle, termite or beagle, passes enough so that the adult has the stored energy to sur through similar stages of development. The stages of devel- vive and mate. Indeed, most female moths mate as soon as they eclose from their pupa, and they fly only once- to lay their eggs. Then they die. Throughout the animal kingdom, an incredible variety 6. In many species, a group of cells is set aside to produce of embryonic types exist, but most pattems of embryogen the next generation(rather than forming the current esis are variations on six fundamental processes: fertiliza- embryo). These cells are the precursors of the gametes tion, cleavage, gastrulation, organogenesis, metamorpho- The gametes and their precursor cells are collectively sis,and gametogenesis. called germ cells, and they are set aside for reproduc tive function. All the other cells of the body are called 1. Fertilization involves the fusion of the mature sex cells, somatic cells. This separation of somatic cells(which the sperm and egg, which are collectively called the to the individual body) and gametes. The fusion of the gamete cells stimulates the contribute to the formation of a new generation) is often gg to begin development and initiates a new individ one of the first differentiations to occur during animal ual. The subsequent fusion of the gamete nuclei (both development. The germ cells eventually migrate to the of which have only half the normal number of chromo gonads, where they differentiate into gametes. The somes characteristic for the species) gives the embryo development of gametes, called gametogenesis, is usu its genome, the collection of genes that helps instruct lly not completed until the organism has become phys. the embryo to develop in a manner very similar to that ically mature. At maturity, the gametes may be released of it parents and participate in fertilization to begin a new embryo 2. Cleavage is a series of extremely rapid mitotic divisions The adult organism eventually undergoes senescence that immediately follow fertilization. During cleavage, and dies, its nutrients often supporting the early the enormous volume of zygote cytoplasm is divided embryogenesis of its offspring and its absence allowing into numerous smaller cells called blastomeres. By the less competition. Thus, the cycle of life is renewed end of cleavage, the blastomeres have usually formed 3.sphere, known as a blastula .fter the rate of mitotic division slows A Frogs Life tomeres undergo dramatic movements and change their positions relative to one another. This series of exten- All animal life cycles are modifications of the generalized one described above. Figure 1. 1 shows the development of sive cell rearrangements is called gastrulation, and the the leopard frog, Rana pipiens, and provides a good start- embryo is said to be in the gastrula stage. As a result of gastrulation, the embryo contains three germ layers that ing point for a more detailed discussion of a representa- will interact to generate the organs of the bod tive life cycle 4. Once the germ layers are established, the cells interact with one another and rearrange themselves to produce Gametogenesis and fertilization tissues and organs. This process is called organogene- The end of one life cycle and the beginning of the next are sis. Many organs contain cells from more than one germ often intricately intertwined. Life cycles are often controlle layer,and it is not unusual for the outside of an organ by environmental factors(tadpoles wouldn't survive if to be derived from one layer and the inside from anoth- er. For example, the outer layer of skin(epidermis they hatched in the fall, when their food is dying), so in omes from the ectoderm, whereas the inner layer(the most frogs, gametogenesis and fertilization are seasonal dermis)comes from the mesoderm. Also during organo- events. A combination of photoperiod (hours of daylight genesis, certain cells undergo long migrations from their and temperature informs the pituitary gland of the mature place of origin to their final location. These migrating mones that stimulate her ovary to make the hormone estro- pigment cells, and sex cells gen Estrogen then instructs the liver to make and secrete yolk proteins, which are then transported through the
CHAPTER 1 development is continually expanded and enhanced by revolutions in microscopy, computer-aided graphical reconstructions of three-dimensional objects, and methods of applying mathematics to biology. Many of the beautiful photographs in this book reflect this increasingly important component of embryology. The Cycle of Life One of the major triumphs of descriptive embryology was the idea of a generalizable animal life cycle. Each animal, whether earthworm or eagle, termite or beagle, passes through similar stages of development. The stages of development between fertilization and hatching are collectively called embryogenesis. Throughout the animal kingdom, an incredible variety of embryonic types exist, but most patterns of embryogenesis are variations on six fundamental processes: fertilization, cleavage, gastrulation, organogenesis, metamorphosis, and gametogenesis. 1. Fertilization involves the fusion of the mature sex cells, the sperm and egg, which are collectively called the gametes. The fusion of the gamete cells stimulates the egg to begin development and initiates a new individual. The subsequent fusion of the gamete nuclei (both of which have only half the normal number of chromosomes characteristic for the species) gives the embryo its genome, the collection of genes that helps instruct the embryo to develop in a manner very similar to that of it parents. 2. Cleavage is a series of extremely rapid mitotic divisions that immediately follow fertilization. During cleavage, the enormous volume of zygote cytoplasm is divided into numerous smaller cells called blastomeres. By the end of cleavage, the blastomeres have usually formed a sphere, known as a blastula. 3. After the rate of mitotic division slows down, the blastomeres undergo dramatic movements and change their positions relative to one another. This series of extensive cell rearrangements is called gastrulation, and the embryo is said to be in the gastrula stage. As a result of gastrulation, the embryo contains three germ layers that will interact to generate the organs of the body. 4. Once the germ layers are established, the cells interact with one another and rearrange themselves to produce tissues and organs. This process is called organogenesis. Many organs contain cells from more than one germ layer, and it is not unusual for the outside of an organ to be derived from one layer and the inside from another. For example, the outer layer of skin (epidermis) comes from the ectoderm, whereas the inner layer (the dermis) comes from the mesoderm. Also during organogenesis, certain cells undergo long migrations from their place of origin to their final location. These migrating cells include the precursors of blood cells, lymph cells, pigment cells, and sex cells. 5. In many species, the organism that hatches from the egg or is born into the world is not sexually mature. Rather, the organism needs to undergo metamorphosis to become a sexually mature adult. In most animals, the young organism is a called a larva, and it may look significantly different from the adult. In many species, the larval stage is the one that lasts the longest, and is used for feeding or dispersal. In such species, the adult is a brief stage whose sole purpose is to reproduce. In silkworm moths, for instance, the adults do not have mouthparts and cannot feed; the larvae must eat enough so that the adult has the stored energy to survive and mate. Indeed, most female moths mate as soon as they eclose from their pupa, and they fly only once— to lay their eggs. Then they die. 6. In many species, a group of cells is set aside to produce the next generation (rather than forming the current embryo). These cells are the precursors of the gametes. The gametes and their precursor cells are collectively called germ cells, and they are set aside for reproductive function. All the other cells of the body are called somatic cells. This separation of somatic cells (which give rise to the individual body) and germ cells (which contribute to the formation of a new generation) is often one of the first differentiations to occur during animal development. The germ cells eventually migrate to the gonads, where they differentiate into gametes. The development of gametes, called gametogenesis, is usually not completed until the organism has become physically mature. At maturity, the gametes may be released and participate in fertilization to begin a new embryo. The adult organism eventually undergoes senescence and dies, its nutrients often supporting the early embryogenesis of its offspring and its absence allowing less competition. Thus, the cycle of life is renewed. A Frog's Life All animal life cycles are modifications of the generalized one described above. Figure 1.1 shows the development of the leopard frog, Rana pipiens, and provides a good starting point for a more detailed discussion of a representative life cycle. Gametogenesis and fertilization The end of one life cycle and the beginning of the next are often intricately intertwined. Life cycles are often controlled by environmental factors (tadpoles wouldn't survive if they hatched in the fall, when their food is dying), so in most frogs, gametogenesis and fertilization are seasonal events. A combination of photoperiod (hours of daylight) and temperature informs the pituitary gland of the mature female frog that it is spring. The pituitary then secretes hormones that stimulate her ovary to make the hormone estrogen. Estrogen then instructs the liver to make and secrete yolk proteins, which are then transported through the
Morula Blastula Oocl Location of Germ plasm germ cells Blastocoel e gamete) Oocyte FERTILI CLEAVAGE ZATION GAMETOGENESIS GASTRULATION MATURITY Ectoderm mature adult Metamorphosis Mesoderm (in some species) ORGANOGENESIS Endoderm LARVAL Gonad STAGES Hatching Immature larval (birth) 1.1 Developmental history of the leopard frog, Rana The stages from fertilization through hatching (birth)are collectively as embryogenesis. The region set aside for pro- cells is shown in purple. Gametogenesis, which is Fertilization accomplishes several things. First, it allows in the sexually mature adult, begins at different times the haploid nucleus of the egg(the female pronucleus)to evelopment, depending on the species. (The sizes of the merge with the haploid nucleus of the sperm(the male ed wedges shown here are arbitrary and do not corre. pronucleus)to form the diploid zygote nucleus. Second, the proportion of the life cycle spent in each stage.) fertilization causes the cytoplasm of the egg to move such that different parts of the cytoplasm find themselves in new locations(Figure 1.2D). This cytoplasmic migration into the enlarging eggs in the ovary. The yolk is will be important in determining the three embryonic axes orted into the bottom portion of the egg, called the of the frog: anterior-posterior(head-tail), dorsal-ventral stal hemisphe here it will serve as food for the ack-belly), and right-left. Third, fertilization activates ng embryo(Figure 1.2A). The upper half of the egg those molecules necessary to begin cell cleavage and gas the animal hemisphere. "Sperm formation also trulation(Rugh 1950) on a seasonal basis. Male leopard frogs make sperm the summer, and by the time they begin hiberna the fall they have produced all the sperm that will Cleavage and gastrulation ble for the following springs breeding season. During cleavage, the volume of the frog egg stays the same, t species of frogs, fertilization is external. The but it is divided into tens of thousands of cells(Figure rog grabs the female's back and fertilizes the eggs as 1. 2E-H). The cells in the animal hemisphere of the egg le releases them(Figure 1. 2B). Some species lay divide faster than those in the vegetal hemisphere, and the eggs in pond vegetation, and the egg jelly adheres to cells of the emi isphere become progressively larg ts and anchors the eggs(Figure 1.2C). Other species er the more vegetal the cytoplasm. Meanwhile, a fluid-fille eggs into the center of the pond without any cavity, the blastocoel, forms in the animal hemisphere( Fig So the first important thing to remember about ure 1.21). This cavity will be important for allowing cell that they are often intimately involved with movements to occur during gastrulation. ntal factors Gastrulation in the frog begins at a point on the embryo surface roughly 180 degrees opposite the point of sperm the terms animal and vegetal for the upper and lower entry with the formation of a dimple, called the blastopore of the early frog embryo reflect the division rates of This dimple(which will mark the future dorsal side of the he upper cells divide rapidly and become actively mobile embryo)expands to become a ring, and cells migrating ted, while the yolk-filled cells of the lower half through the blastopore become the mesoderm( Figure pen as being immobile(hence like plants, or"vegetal") 13A-C). The cells remaining on the outside become the ecto-
UCVtLUnvitlMA L AINAIUMY : 1.1 Developmental history of the leopard frog, Rana .The stages from fertilization through hatching (birth) are collectively as embryogcnesis. The region set aside for pro- - - :n cells is shown in purple. Cametogenesis, which is fed in the sexually mature adult, begins at different times development, depending on the species. (The sizes of the wed wedges shown here arc arbitrary and do not corre- > the proportion of the life cycle spent in each stage.) i into the enlarging eggs in the ovary. The yolk is : acted into the bottom portion of the egg, called the al hemisphere, where it will serve as food for the krping embryo (Figure 1.2A). The upper half of the egg fed the animal hemisphere.* Sperm formation also i on a seasonal basis. Male leopard frogs make sperm I ; the summer, and by the time they begin hibernar. the fall they have produced all the sperm that will iflable for the following spring's breeding season. •ost species of frogs, fertilization is external. The rog grabs the female's back and fertilizes the eggs as male releases them (Figure 1.2B). Some species lay :eggs in pond vegetation, and the egg jelly adheres to sr.ts and anchors the eggs (Figure 1.2C). Other species tr.eir eggs into the center of the pond without any DTL So the first important thing to remember about fries is that they are often intimately involved with xrrjnental factors. of the terms animal and vegetal for the upper and lower ; of the early frog embryo reflect the division rates of The upper cells divide rapidly and become actively mobile nimated"), while the yolk-filled cells of the lower half : as being immobile (hence like plants, or "vegetal"). Fertilization accomplishes several things. First, it allows the haploid nucleus of the egg (the female pronucleus) to merge with the haploid nucleus of the sperm (the male pronucleus) to form the diploid zygote nucleus. Second, fertilization causes the cytoplasm of the egg to move such that different parts of the cytoplasm find themselves in new locations (Figure 1.2D). This cytoplasmic migration will be important in determining the three embryonic axes of the frog: anterior-posterior (head-tail), dorsal-ventral (back-belly), and right-left. Third, fertilization activates those molecules necessary to begin cell cleavage and gastrulation (Rugh 1950). Cleavage and gastrulation During cleavage, the volume of the frog egg stays the same, but it is divided into tens of thousands of cells (Figure 1.2E-H). The cells in the animal hemisphere of the egg divide faster than those in the vegetal hemisphere, and the cells of the vegetal hemisphere become progressively larger the more vegetal the cytoplasm. Meanwhile, a fluid-filled cavity, the blastocoel, forms in the animal hemisphere (Figure 1.21). This cavity will be important for allowing cell movements to occur during gastrulation. Gastrulation in the frog begins at a point on the embryo surface roughly 180 degrees opposite the point of sperm entry with the formation of a dimple, called the blastopore. This dimple (which will mark the future dorsal side of the embryo) expands to become a ring, and cells migrating through the blastopore become the mesoderm (Figure 1.3A-C). The cells remaining on the outside become the ecto-