226 ESSENTIALS OF ZOOLOGY 



a segmentation cavity. The condition is almost exactly 

 that of Amphioxus ; but invagination is impossible, and the 

 cavity is narrowed by the delamination of cells into the interior. 

 Most are formed by the endoderm, but, as Samasso's experiment 

 indicated, they arise also from the margin of the ectoderm 

 and endoderm. The segmentation cavity is thereby converted 

 into an enteron. The subsequent events show, however, that 

 the delamination resorted to is not strictly comparable to that 

 of the Coelenterates, but is a process brought about by the 

 increase in yolk of the endoderm cells at the vegetable pole 

 preventing invagination. A blastopore is formed at the margin 

 of the ectoderm and the endoderm that is, about midway 

 between the two poles of the egg. And it is widely distended 

 by the yolk cells of the endoderm. It will be observed that 

 the condition is similar to that of the Elasmobranch, but the 

 yolk is so massive in the egg of the Elasmobranch as to bring 

 the blastopore close to the animal pole. 



The margin of the blastopore advances over the yolk cells, 

 the process beginning at the dorsal lip and extending to the 

 ventral. The blastoporal rim gradually narrows, therefore, 

 as it approaches the vegetable pole, which is the posterior 

 pole. This pole of the egg is white, and the yolk cells stand 

 out in relief as the darkly pigmented ectoderm advances 

 over them. The narrowing white area is called the yolk 

 plug. It is reduced to a white spot, and is finally obliterated 

 with the formation of the neurenteric canal. Gastrulation 

 may be said to be ended, therefore, as soon as delamination 

 has taken place and the margin of the blastopore has appeared. 

 The subsequent changes relate to the postgastrula. 



The backward growth of the dorsal lip gives rise to an 

 extension of ectoderm and endoderm and the formation of 

 mesoderm. An enteron is thus formed immediately underneath 

 the dorsal lip, and it is extended by the growth of this region 

 of the blastopore. But it also excavates its way among the 

 delaminated endoderm. The endoderm at the same time 

 gradually encloses the primitive enteron. When the blastopore 

 approaches the posterior end of the egg the enteron undergoes 

 a large expansion, one cavity resulting. The primitive 

 enteron is said by some observers to disappear in the 



