The mesoderm forms most skeletal and muscular tissues, the circulatory system, and urinary and reproductive organs (see Figure 8.26 ). As vertebrates have increased in size and complex-ity, mesodermally derived supportive, movement, and transport structures have become an even greater proportion of the body.
Most muscles arise from the mesoderm along each side of the neural tube ( Figure 8.30 ). This mesoderm divides into a lin-ear series of blocklike somites (38 in humans), which by split-ting, fusion, and migration become the axial skeleton, dermis of the dorsal skin, and muscles of the back, body wall, and limbs.
Mesoderm gives rise to the fi rst functional organ, the embry-onic heart. Guided by the underlying endoderm, two clusters of precardiac mesodermal cells move amebalike into position on either side of the developing gut. These clusters differentiate into a pair of double-walled tubes, which later fuse to form a single, thin tube (see Figure 8.14 , p. 168).
As the cells group together, the fi rst twitchings are evi-dent. In a chick embryo, a favorite animal for experimental embryological studies, the primitive heart begins to beat on the second day of the 21-day incubation period; it begins beating before any true blood vessels have formed and before there is any blood to pump. As the ventricle primordium develops, the spontaneous cellular twitchings become coordinated into a feeble but rhythmical beat. New heart chambers, each with a beat faster than its predecessor, then develop.
Figure 8.29
Derivatives of the alimentary canal of a human embryo.
Brain Thyroid gland
Notochord Thymus Trachea Stomach Pancreas Intestine
Anus Liver Mouth
Esophagus Somites
Gill pouches
Front limb bud
Hind limb bud Umbilical
cord
Post-anal tail
Figure 8.30
Human embryo showing somites, which differentiate into skeletal muscles and axial skeleton.
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Early development of the heart and circulation is crucial to continued embryonic development, because without circu-lation an embryo could not obtain materials for growth. Food is absorbed from the yolk and carried to the embryonic body, oxygen is delivered to all tissues, and carbon dioxide and other wastes are carried away. An embryo is totally dependent on these extraembryonic support systems, and the circulation is the vital link between them.
S U M M A R Y
Developmental biology encompasses the emergence of order and complexity during the development of a new individual from a fertilized egg, and the control of this process. The early prefor-mation concept of development gave way in the eighteenth cen-tury to the theory of epigenesis, which holds that development is the progressive appearance of new structures that arise as the products of antecedent development. Fertilization of an egg by a sperm restores the diploid number of chromosomes and acti-vates the egg for development. Both sperm and egg have evolved devices to promote effi cient fertilization. The sperm is a highly condensed haploid nucleus provided with a locomotory fl agel-lum. Many eggs release chemical sperm attractants, most have surface receptors that recognize and bind only with sperm of their own species, and all have developed devices to prevent polyspermy.
During cleavage an embryo divides rapidly and usually syn-chronously, producing a multicellular blastula. Cleavage is greatly infl uenced by quantity and distribution of yolk in the egg. Eggs with little yolk, such as those of many marine invertebrates, divide com-pletely (holoblastic) and usually have indirect development with a larval stage interposed between the embryo and adult. Eggs having an abundance of yolk, such as those of birds, reptiles, and most arthropods divide only partially (meroblastic), and birds and reptiles have no larval stage.
Based on several developmental characteristics, bilateral meta-zoan animals are divided into two major groups. The Protostomia have mosaic cleavage and the mouth forms at or near the embryonic blastopore. The Deuterostomia have regulative cleavage and the mouth forms secondarily and not from the blastopore.
At gastrulation, cells on an embryo’s surface move inward to form germ layers (endoderm, ectoderm, mesoderm) and the embry-onic body plan. Like cleavage, gastrulation is much infl uenced by the quantity of yolk.
Despite the different developmental fates of embryonic cells, every cell contains a complete genome and thus the same nuclear information. Early development through cleavage is governed by cytoplasmic determinants derived from the maternal genome and placed in the egg cortex. As gastrulation approaches, control gradu-ally shifts from maternal to embryonic as an embryo’s own nuclear genes begin transcribing mRNA.
Harmonious differentiation of tissues proceeds in three gen-eral stages: pattern formation, determination of position in the body, and induction of limbs and organs appropriate for each position. Each stage is guided by morphogens. Pattern formation refers to determination of the anteroposterior, dorsoventral, and left-to-right body axes. In amphibians the anteroposterior axis is
established by morphogens such as chordin from the Spemann organizer in the gray crescent of the zygote. In Drosophila that axis is determined by the morphogen bicoid, which is transcribed from maternal mRNA deposited at the anterior of the egg. In these and other segmented animals, such morphogens activate genes that divide the body into head, thorax and abdomen, and then into correctly oriented segments. The structures appropriate to each segment are then induced by homeotic genes, which are characterized by a particular sequence of DNA bases called the homeobox. Mutations in homeotic genes result in the develop-ment of inappropriate structures on a segdevelop-ment: legs on the head, for example.
The anteroposterior axis of an embryo is determined by homeo-tic and other homeobox-containing genes contained in one or more clusters on particular chromosomes. These genes, called Hox genes, occur not only in Drosophila and amphibians, but apparently in all animals. Each Hox gene is active in a particular region of the body, depending on its position within the cluster. Dorsoventral and left-right axes are similarly determined by morphogens that are produced only in the appropriate regions of the embryo. Similarly, morphogens guide the development of limbs along three body axes.
Morphogens have been found to be remarkably similar in animals as different as Drosophila and amphibians. This realization has given rise to the fi eld of evolutionary developmental biology, which is based on the idea that the evolution of the enormous variety of ani-mals is the result of changes in the position and timing of relatively few genes that control development.
The postgastrula stage of vertebrate development represents a remarkable conservation of morphology when jawed vertebrates from fi sh to humans exhibit features common to all. As development proceeds, species-specifi c characteristics are formed.
Amniotes are terrestrial vertebrates that develop extraembry-onic membranes during embryextraembry-onic life. The four membranes are amnion, allantois, chorion, and yolk sac, each serving a specifi c life-support function for the embryo that develops within a self-contained egg (as in birds and most reptiles) or within the maternal uterus (mammals).
Mammalian embryos are nourished by a placenta, a complex fetal-maternal structure that develops in the uterine wall. During pregnancy the placenta becomes an independent nutritive, endo-crine, and regulatory organ for the embryo.
Germ layers formed at gastrulation differentiate into tissues and organs. The ectoderm gives rise to skin and nervous system;
endoderm gives rise to alimentary canal, pharynx, lungs, and cer-tain glands; and mesoderm forms muscular, skeletal, circulatory, reproductive, and excretory organs.
Finally a specialized area of heart muscle called the sino-atrial (SA) node develops and takes command of the entire heartbeat (the role of the SA node in the excitation of the heart is described on p. 694). The SA node becomes the heart’s pri-mary pacemaker. As the heart builds a strong and effi cient beat, vascular channels open within the embryo and across the yolk.
Within the vessels are the fi rst primitive blood cells suspended in plasma.
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w w w . m h h e . c o m / h i c k m a n i p z 1 4 e CHAPTER 8 Principles of Development 183
R E V I E W Q U E S T I O N S
What is meant by epigenesis? How did Kaspar Friedrich Wolff’s concept of epigenesis differ from the early notion of preformation?
How is an egg (oocyte) prepared during oogenesis for fertilization? Why is preparation essential to development?
Describe events that follow contact of a spermatozoon with an egg. What is polyspermy and how is it prevented?
What is meant by the term “activation” in embryology?
How does amount of yolk affect cleavage? Compare cleavage in a sea star with that in a bird.
What is the difference between radial and spiral cleavage?
What other developmental hallmarks are often associated with spiral or radial cleavage?
What is indirect development?
Using sea star embryos as an example, describe gastrulation.
Explain how the mass of inert yolk affects gastrulation in frog and bird embryos.
What is the difference between schizocoelous and enterocoelous origins of a coelom?
Describe two different experimental approaches that serve as evidence for nuclear equivalence in animal embryos.
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What is meant by “induction” in embryology? Describe the famous organizer experiment of Spemann and Mangold and explain its signifi cance.
What are homeotic genes and what is the “homeobox”
contained in such genes? What is the function of the homeobox? What are Hox genes? What is the signifi cance of their apparently universal occurrence in animals?
What is the embryological evidence that vertebrates form a monophyletic group?
What are the four extraembryonic membranes of amniotic eggs of birds and reptiles and what is the function of each membrane?
What is the fate of the four extraembryonic membranes in embryos of placental mammals?
Explain what the “growth cone” that Ross Harrison observed at the ends of growing nerve fi bers does to infl uence direction of nerve growth.
Name two organ system derivatives of each of the three germ layers.
What developmental characters are used to divide animals between protostome and deuterostome groups (clades)?
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S E L E C T E D R E F E R E N C E S
Carroll, S. B., J. K. Grenier, and S. D. Weatherbee. 2005. From DNA to diversity: molecular genetics and the evolution of animal design, ed.
2. Malden, Massachusetts, Blackwell Publishing. Animal body plans develop through a hierarchy of gene interactions. As these interactions are understood, biologists seek commonalities in the “genetic toolkit”
across a wide variety of taxa.
Cibelli, J. B., R. P. Lanza, and M. D. West. 2002. The fi rst human cloned embryo. Sci. Am. 286: 44–51 (Jan.). Describes the fi rst cloning of human embryos—but only to the 6-cell stage. Many scientists remain skeptical.
Degnan, B. M., S. P. Leys, and C. Larroux. 2005. Sponge development and antiquity of animal pattern formation. Integr. Comp. Biol. 45: 335–341.
After blastula formation in a demosponge embryo, cell migration produces a two-layered gastrula that develops a third layer before becoming a free-swimming larva. If this pattern is typical, it suggests that both blastula and gastrula stages were present in ancestral metazoans.
Gilbert, S. F. 2003. Developmental biology, ed. 7. Sunderland, Massachusetts, Sinauer Associates. Combines descriptive and mechanistic aspects; good selection of examples from many animal groups.
Gilbert, S. F., and A. M. Raunio (eds.). 1997. Embryology: constructing the organism. Sunderland, Massachusetts, Sinauer Associates. The embryology of numerous animal groups.
Goodman, C. S., and M. J. Bastiani. 1984. How embryonic nerve cells recognize one another. Sci. Am. 251: 58–66 (Dec.). Research with insect larvae shows that developing neurons follow pathways having specifi c molecular labels.
Leys, S. P., and D. Eerkes-Medrano. 2005. Gastrulation in calcareous sponges:
in search of Haeckel’s gastraea. Integr. Comp. Biol. 45: 342–351.
Ingression of cells during embryogenesis produces two germ layers (interpreted as gastrulation) in Sycon, a calcareous sponge. The ancestral pattern of gastrulation may be via ingression rather than by invagination, which occurs much later during larval metamorphosis.
Nüsslein-Volhard, C. 1996. Gradients that organize embryo development.
Sci. Am. 275: 54–61 (Aug.). An account of the author’s Nobel Prize–
winning research.
Rosenberg, K. R., and W. R. Trevathan. 2001. The evolution of human birth.
Sci. Am. 285: 72–77 (Nov.). Examines reasons why humans are the only primates to seek assistance during childbirth.
Wolpert, L. 1991. The triumph of the embryo. Oxford, Oxford University Press. Written for the nonspecialist, this engaging book is rich in detail and insight for all biologists interested in the development of life.
O N L I N E L E A R N I N G C E N T E R
Visit www.mhhe.com/hickmanipz14e for chapter quizzing, key term fl ash cards, web links and more.
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3
Diversity of Animal Life
P A R T T H R E E
A view of coral reef biodiversity.
9 Architectural Pattern of an Animal 10 Taxonomy and Phylogeny of Animals 11 Protozoan Groups
12 Mesozoa and Parazoa 13 Radiate Animals
14 Acoelomate Bilateral Animals 15 Pseudocoelomate Animals 16 Molluscs
17 Annelids and Allied Taxa 18 Smaller Ecdysozoans
19 Trilobites, Chelicerates, and Myriapods 20 Crustaceans
21 Hexapods
22 Chaetognaths, Echinoderms, and Hemichordates
23 Chordates 24 Fishes
25 Early Tetrapods and Modern Amphibians 26 Amniote Origins and Nonavian Reptiles 27 Birds
28 Mammals
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