THE ONTOGENESIS OF VERTEBRATES 59 



toats freely in the water, as in that of many of the inverte- 

 brates, each cell is provided with long vibratile flagella, by 

 which the colony is moved. This larval form is closely imi- 

 tated by such an organism as Volvox, which is usually reck- 

 oned as a plant, but serves to show a physiologically functional 

 adult organism in the corresponding stage. The folding in, 

 or collapse of one portion of the blastula, .as in the diagram, 



I 



o 



II 



III 



Yin 



FIG. 12. Early Metazoan development; typical. [After models of Am- 

 phioxus by HATSCHEK.] 



I, the egg. II, III, and IV, cleavage stages. V and VI, blastula: in VI, which 

 represents a somewhat older stage than V; one-half has been removed. VII repre- 

 sents the beginning of the gastrular invagation, and VIII is the completed gastrula, 

 both sectioned as in VI. 



produces a two-layered cup which forms the next important 

 ontogenetic stage, the gastrula, and in attaining this the 

 embryo passes beyond the Protozoa in its imitative repetition 

 and assumes the essential form of the simplest of the Metazoa, 

 the Ccelenterata. A typical gastrula is radiate in structure, and 

 possesses a central axis with two poles, oral and apical, the 

 former characterized by the presence of the gastrula mouth 

 or protostome. This latter leads into the large central cavity, 

 the gastroccele, which has developed from the exterior at the 

 expense of the cavity of the blastula, the blastocccle. In some 



