252 



Basic Structure of Vertebrates 



Gastrula 



In Amphioxus. The blastula stage is briefly transitory. At once 

 changes set in which transform it to a two-layer embryo. In Amphioxus 

 the two-layer gastrula form is attained in a very simple way (Fig. 

 210). The vegetal hemisphere first flattens, then becomes curved in- 

 ward. The infolding (invagination) continues until the material of 

 the original vegetal hemisphere comes into close relation with the 

 inner surface of the wall of the animal hemisphere. The spheric blastula 

 thus becomes an approximately hemispheric embryo whose wall con- 

 sists of two layers (Fig. 210C), an ectoderm and an endoderm. As 

 the process goes on, the blastocoele is reduced and finally obliterated. 

 The gastrula is hollow. Its cavity, resulting from the invagination, 

 is the primary digestive cavity or archenteron. At first it opens 



ANIMAL POLE 



Fig. 210. Gastrulation in Amphioxus. The figures represent sections through 

 the polar axis of the embryo. (A) Blastula with vegetal region flattened. (B, C) 

 Earlier and later stages of invagination of vegetal hemisphere. (D) Gastrulation 

 completed; with elongation of the gastrula, its long axis becomes the horizontal 

 anteroposterior axis of the embryo, (a) Archenteron; (b) blastocoele; (bp) blasto- 

 pore; (ec) ectoderm; (en) endoderm; (p) polar body. (After Cerfontaine. 

 Courtesy, Neal and Band: "Chordate Anatomy," Philadelphia, The Blakiston 

 Company.) 



