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Gilbert SF. Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates; 2000.

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Developmental Biology. 6th edition.

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Developmental Patterns among the Metazoa

Since the remainder of this book concerns the development of metazoans—multicellular animals* that pass through embryonic stages of development—we will present an overview of their developmental patterns here. Figure 2.21 illustrates the major evolutionary trends of metazoan development. The most striking pattern is that life has not evolved in a straight line; rather, there are several branching evolutionary paths. We can see that metazoans belong to one of three major branches: Diploblasts, protostomes, and deuterostomes.

Figure 2.21. Major evolutionary divergences in extant animals.

Figure 2.21

Major evolutionary divergences in extant animals. Other models of evolutionary relationships among the phyla are possible.This grouping of metazoa is based on embryonic, morphological, and molecular criteria. (Based on J. R. Garey, personal communication.) (more...)

Sponges develop in a manner so different from that of any other animal group that some taxonomists do not consider them metazoans at all, and call them “parazoans.” A sponge has three major types of somatic cells, but one of these, the archeocyte, can differentiate into all the other cell types in the body. Individual cells of a sponge passed through a sieve can reaggregate to form new sponges. Moreover, in some instances, such reaggregation is species-specific: if individual sponge cells from two different species are mixed together, each of the sponges that re-forms contains cells from only one species (Wilson 1907). In these cases, it is thought that the motile archeocytes collect cells from their own species and not from others (Turner 1978). Sponges contain no mesoderm, so the Porifera have no true organ systems; nor do they have a digestive tube or circulatory system, nerves, or muscles. Thus, even though they pass through an embryonic and a larval stage, sponges are very unlike most metazoans (Fell 1997). However, sponges do share many features of development (including gene regulatory proteins and signaling cascades) with all the other animal phyla, suggesting that they share a common origin (Coutinho et al. 1998).

Diploblasts

Diploblastic animals are those who have ectoderm and endoderm, but no true mesoderm. These include the cnidarians (jellyfish and hydras) and the ctenophores (comb jellies).

Cnidrians and ctenophores constitute the Radiata, so called because they have radial symmetry, like that of a tube or a wheel. In these animals, the mesoderm is rudimentary, consisting of sparsely scattered cells in a gelatinous matrix.

Protostomes and deuterostomes

Most metazoans have bilateral symmetry and three germ layers. The animals of these phyla, known collectively as the Bilatera, are classified as either protostomes or deuterostomes. All Bilateria are thought to have descended from a primitive type of flatworm. These flatworms were the first to have a true mesoderm (although it was not hollowed out to form a body cavity), and they may have resembled the larvae of certain contemporary coelenterates.

There are two divisions of bilaterian phyla, the protostomes and the deuterostomes. Protostomes (Greek, “mouth first”), which include the mollusc, arthropod, and worm phyla, are so called because the mouth is formed first, at or near the opening to the gut, which is produced during gastrulation. The anus forms later at another location. The coelom, or body cavity, of these animals forms from the hollowing out of a previously solid cord of mesodermal cells.

There are two major branches of the protostomes. The ecdysozoa includes those animals that molt. Its major constituent is Arthropoda, a phylum containing insects, arachnids, mites, crustaceans, and millipedes. The second major group of protostomes are the lophotrochozoa. They are characterized by a common type of cleavage (spiral), a common larval form, and a distinctive feeding apparatus. These phyla include annelids, molluscs, and flatworms.

Phyla in the deuterostome lineage include the chordates and echinoderms. Although it may seem strange to classify humans, fish, and frogs in the same group as starfish and sea urchins, certain embryological features stress this kinship. First, in deuterostomes (“mouth second”), the mouth opening is formed after the anal opening. Also, whereas protostomes generally form their body cavities by hollowing out a solid mesodermal block (schizocoelous formation of the body cavity), most deuterostomes form their body cavities from mesodermal pouches extending from the gut (enterocoelous formation of the body cavity). It should be mentioned that there are many exceptions to these generalizations.

The evolution of organisms depends on inherited changes in their development. One of the greatest evolutionary advances—the amniote egg—occurred among the deuterostomes. This type of egg, exemplified by that of a chicken (Figure 2.22), is thought to have originated in the amphibian ancestors of reptiles about 255 million years ago. The amniote egg allowed vertebrates to roam on land, far from existing ponds. Whereas most amphibians must return to water to lay their eggs, the amniote egg carries its own water and food supplies. It is fertilized internally and contains yolk to nourish the developing embryo. Moreover, the amniote egg contains four sacs: the yolk sac, which stores nutritive proteins; the amnion, which contains the fluid bathing the embryo; the allantois, in which waste materials from embryonic metabolism collect; and the chorion, which interacts with the outside environment, selectively allowing materials to reach the embryo. The entire structure is encased in a shell that allows the diffusion of oxygen but is hard enough to protect the embryo from environmental assaults and dehydration. A similar development of egg casings enabled arthropods to be the first terrestrial invertebrates. Thus, the final crossing of the boundary between water and land occurred with the modification of the earliest stage in development: the egg.

VADE MECUM

Egg development. The anatomy of the amniote egg is seen in video. [Click on Chick-Early]

Figure 2.22. Diagram of the amniote egg of the chick, showing the membranes enfolding the 7-day chick embryo.

Figure 2.22

Diagram of the amniote egg of the chick, showing the membranes enfolding the 7-day chick embryo. The yolk is eventually surrounded by the yolk sac, which allows the entry of nutrients into the blood vessels. The chorion is derived in part from the ectoderm (more...)

Embryology provides an endless assortment of fascinating animals and problems to study. In this text, we will use but a small sample of them to illustrate the major principles of animal development. This sample is an incredibly small collection. We are merely observing a small tidepool within our reach, while the whole ocean of developmental phenomena lies before us.

After a brief outline of the experimental and genetic approaches to developmental biology, we will investigate the early stages of animal embryogenesis: fertilization, cleavage, gastrulation, and the establishment of the body axes. Later chapters will concentrate on the genetic and cellular mechanisms by which animal bodies are constructed. Although an attempt has been made to survey the important variations throughout the animal kingdom, a certain deuterostome chauvinism may be apparent. (For a more comprehensive survey of the diversity of animal development across the phyla, see Gilbert and Raunio 1997.)

Footnotes

*

Plants undergo equally complex and fascinating patterns of embryonic and postembryonic development. However, plant development differs significantly from that of animals, and the decision was made to focus this text on the development of animals. Readers who wish to discover some of the differences are referred to Chapter 20, which provides an overview of plant life cycles and the patterns of angiosperm (seed plant) development.

In mammals, the chorion is modified to form the placenta—another example of the modification of development to produce evolutionary change.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, Sinauer Associates.
Bookshelf ID: NBK10054

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