THE REPRODUCTION OF ANIMALS 



233 



ment, however, is much more nearly — though not entirely — like that 

 of a strongly telolecithal egg, and this is undoubtedly related to the fact 

 that the mammals are descended from oviparous ancestors that laid 

 telolecithal eggs. 



Early Stages and Processes of Development 



Except for such modifications and distortions as are evidently due 

 to the varied amount and distribution of yolk, the early embryonic stages 



Fig. 15.13. Cleavage and gastrulation in the development of the starfish. A, the unfertilized 

 homoleeithal egg. B, the fertilized egg with two polar bodies. C, early two-cell stage. D, 

 late two-cell stage. E, four-cell stage. F, eight-cell stage. G, sixteen-cell stage. H, morula 

 ("mulberry") stage. /, blastula, optical section. J, early gastrula, optical section. K, 

 later gastrula, optical section. L, late gastrula, optical section. (Modified from Turtox chart, 

 courtesy General Biological Supply House, Inc.) 



of all Metazoa (except sponges) are clearly similar. The brief account 

 that follows is somewhat generalized but is based primarily upon the 

 development of the homoleeithal eggs of the starfish and the very nearly 

 homoleeithal eggs of Amphioxus. The latter animal (Fig. B.26) is a primi- 

 tive relative of the vertebrates and shows the early development of 

 certain characteristic vertebrate structures. The diagrammatic illustra- 

 tions of the early development of the starfish (Fig. 15.13) and Amphioxus 

 (Fig. 15.14) should be compared with that showing the somewhat modi- 



