COMPARATIVE EMBRYOLOGY 



809 



derm. The processes by which the formation of these definitive germ 

 layers is brought about we speak of as rnesoderm formation. There 

 are really four methods by which gastrulation is accomplished in the 

 animal kingdom : embolic or invaginate gastrulation, epibolic gastrula- 

 tion, gastrulation by polar ingression, and gastrulation by delamina- 

 tion. As in the case of the coeloblastula (Fig. 48), the invaginate 

 gastrulation is the kind people usually think of in following the story 

 of development, but it again is only one of the several types. It is 

 the most simple type, however, and occurs in those embryos which 

 have a cleavage cavity of fair size. The cells at the vegetal pole 

 flatten and gradually push in, so that they come to form a lining layer 

 enclosed by cells of the other half of the egg. The new cavity formed 



Fig. 418. 



Fig. 419. 



Fig. 418. — Morula of the coelenterate, Clava squamata. Notice the "mulberry 

 appearance. (Reprinted by permission from Outline of Comparative Embryology 

 by Richards, published by John Wiley and Sons. Inc., after Harm.) 



Fig-. 419. — Placula of the tunicate, Cynthia partita. (Reprinted by permission 

 from Outline of Comparative Embryology by Richards, published by John Wiley 

 and Sons, Inc., after Conklin. ) 



by their inward movement becomes the gastrocoele or primitive gut 

 cavity, and will develop into the archenteron of the embryo. The best 

 illustration of this type of gastrulation is found in echinoderms, but 

 many others are scattered throughout the animal kingdom. 



Epibolic gastrulation (Fig. 289), on the other hand, is a process of 

 overgrowth occurring in those embryos with very large yolk-contain- 

 ing cells, small ectodermal cells with no cavity between, and spiral 

 cleavage. The ectodermal cells begin a gradual process of overgrowth, 

 which when completed converts the stereogastrula into an embryo con- 

 taining a relatively few primary endodermal cells surrounded by a 

 layer of numerous small primary ectodermal cells. 



