ORIGIN OF GASTRULA 



265 



ovy", ovy'"} the protoplasm of each forming a single ovum : 

 the protoplasm of others divides repeatedly and forms aggre- 

 gations of sperms (spy, spy', spy' 1 }. By the conjugation of a 

 sperm (F) with an ovum (G) an oosperm (H) is produced, 

 and from this by continued division a new colony arises. 



Volvox is clearly comparable to a hollow polyplast, and 

 presents the further resemblance to the higher or multicellular 



MtTi 



fvac 



--Ecb 



nu 



FIG. 67. Diagram illustrating the hypothetical origin of the gastrula 

 from a simple multinucleate form. 



A, infusor-like animal with mouth (Mtk) and gullet, and numerous 

 nuclei (mi) and food vacuoles (f. vac). 



B, similar form in which the food vacuoles have coalesced into a 

 single digestive cavity (f. vac') communicating with the gullet, and the 

 nuclei (nu) are regularly arranged in two layers. 



C, gastrula formed by the division of the protoplasm of B into as 

 many cells as there are nuclei : the cells form ectoderm (Ect) and 

 endoderm (End), and the digestive cavity is a well-defined enteron 

 (Ent). 



animals that certain of its cells are differentiated to form 

 true sexual products. 



Starting from these two forms we may suppose an Amoeba- 

 like organism (Fig. 66, B 1 ) to have given rise to a solid 

 spherical colony (2), something like Magosphsera. From 

 this, by accumulation of fluid in the interior we get a hollow 

 one-layered sphere (z), like a greatly simplified Volvox, and 

 from this by division of each of its cells into two, an inner 



