EMBRYOLOGY OF TADPOLE AND CHICK 



555 



The fact that the entire yolk comes to lie within the blastoderm 

 causes much of the growth process to be hidden from view. In fact, 

 Professor Johnstone of Cambridge University suggests that the frog em- 

 bryo may not have the three regular germ layers at all. We think they 

 are present, although pressed together so closely that it is practically 

 impossible to distinguish them. 



A slight separation of the darker cell layer in the yolk-plug region 

 leaves an opening called the segmentation cavity or blastocoele (Figs. 

 322, 323). The portion of the pigmented layer which has separated from 

 the yolk-plug will now be known as the dorsal lip of the blastopore (Fig. 

 323). 



The segmentation cavity forms near the animal pole. The entire 

 blastula, however, is not much larger than the original egg because, 

 although there are now thirty-two to sixty-four cells definitely formed, 

 these have been formed by constant cell division without much growth 

 after dividing. The roof and walls of the segmentation cavity are, there- 

 fore, composed of the external pigmented animal cells. These cells are 

 of different shapes and sizes, rather irregular and loosely arranged, and 

 really divided into two sheets, one lining the blastocoele, and the other 

 forming the true outer layer of the blastula. It will thus be seen that 



Fig. 323. 



Median sagittal sections through a series of gastrulas of the frog (R. tem- 

 poraria). The figures illustrate the change in position of the whole gastrula, as 

 well as the phenomena of gastrulation proper. A. Commencement of gastrulation ; 

 earliest appearance of the dorsal lip of the blastopore. Internally the gastrular 

 cleavage is indicated. B. Invagination more pronounced; beginning of epiboly. 

 C. Invagination, epiboly and involution in progress. The gastrular cleavage is 

 now indicated on the side opposite the blastopore. Rotation of the gastrula. D. 

 Just before the ventral lip of the blastopore reaches the median line. The in- 

 dentation of the wall of the segmentation cavity is an artifact. E. Blastopore 

 circular and filled with yolk plug. Gastrula beginning to rotate back to its 

 original position. Peristomial mesoderm differentiating. F. Segmentation cavity 

 nearly obliterated. Neural plate established. G. Gastrulation completed. o, 

 Archenteron ; b, blastopore ; c, rudiment of notochord ; ec, ectoderm ; en, endoderm ; 

 gc, gastrular cleavage ; ge, gut endoderm ; m, peristomial mesoderm ; np, neural 

 plate ; nt, transverse neural ridge ; s, segmentation cavity or blastocoele. (After 

 Brachet.) 



