The Questions of Developmental Biology

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.