o 



FIG. 53. Figures illustrating the cleavage of a holoblastic egg, and the formation of germ 

 layers. A-K, cleavage and formation of the blastula ; L-M, gastrulation ; N, pro- 

 duction of the mesoderm and ccelomic cavities ; O, coelom further developed ; ak, 

 ectoderm; dh, primitive alimentary canal; ik, entoderm ; mk\, somatic layer of 

 mesoderm ; mkz, splanchnic layer of mesoderm. (From Korschelt and Heider after 

 Hatschek.) 



