88 



COLLEGE ZOOLOGY 



FIG. 51. Figures illustrating the cleavage of the holoblastic egg of Am- 

 phioxus, and the formation of germ layers. A-K, cleavage and formation of 

 the blastula. L-M, gastrulation. N, production of the mesoderm' and 

 ccelomic cavities. O, coelom further developed, ak, ectoderm; dh, primitive 

 alimentary canal; ik, entoderm; mk lt somatic layer of mesoderm; mk z , 

 splanchnic layer of mesoderm. (From Korschelt and Heider, after Hatschek.) 



