CREATION BY EVOLUTION
or during fertilization all eggs produce two polar bodies, or rudimentary eggs, which undergo no further development; that in all animals fertilization occurs in essentially the same way; that all eggs undergo a series of divisions or cleavages which lead to the formation of a hollow sphere, the blastula, and that the blastula gets a digestive cavity and becomes a two-layered embryo, the gastrula, comparable to a simple sponge or hydroid. It is certainly no accident that the cleavage of the egg in types as distinct as flatworms, annelids, and moUusks is almost cell for cell the same, and that, where differences exist, rudimentary cells have been found that correspond to well-developed cells in other forms. It is no accident that the eggs of all chordates, for example, have the same type of organization, that all develop a notochord and nervous system and sense organs and gill slits and excretory organs and hundreds of other structures in essentially the same way. These fundamental resemblances, or homologies, as they are technically called, call for some explanation, and the only natural explanation that has ever been proposed is evolution.
Some of these homologies between different embryos and between these and adult forms of lower animals are worthy of more detailed mention. The earliest stages of ontogeny are in many respects like the lowest living organisms. The egg carries us back to the protozoan; cleavage recalls the protozoan colony; the blastula suggests volvox-like forms; the radial gastrula suggests the sponge or hydroid forms; and the bilateral gastrula suggests polyclad-like forms.
From the gastrula stage onward different phyla usually follow different paths, but all the members of each phylum show many fundamental resemblances in embryonic and larval stages, though they may differ notably in adult structure. For example, crustaceans that differ widely in adult
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