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 excre- 

 tory organs and hundreds of other structures in essentially 

 the same way. These fundamental resemblances, or homol- 

 ogies, as they are technically called, call for some explana- 

 tion, 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 struc- 

 ture. For example, crustaceans that differ widely in adult 



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