56 TEXT-BOOK OF EMBRYOLOGY. 



from the smaller cells. The groove represents the beginning of a process of 

 invagination which, however, is much less conspicuous than that in Amphi- 

 oxus where the whole side of the blastula is invaginated. In the frog the 

 yolk cells, laden with inert substance, are much less yielding to such factors 

 as would produce invagination. 



The successive stages of gastrulation as seen in sagittal section can be 

 followed clearly in Fig. 31. The pictures are more vivid than verbal des- 

 cription. The groove can be seen to grow deeper in successive stages, turn- 

 ing upward into the elevation of yolk cells, seeming to push that elevation 

 before it, and following the roof of the blastoccel across to the opposite side. 

 When well on its way, the groove expands into a broad space which finally 

 occupies the interior of the structure in much the same way as did the blasto- 



FIG. 30. Diagrams showing the position of the blastopore at successive stages of gastrulation 

 in the frog's egg. A, posterior view; B, lateral view. Figures 1-5 indicate the shape and 

 position of the blastopore during the internal changes; figure 5 indicates its position after 

 the rotation of the gastrula. Compare Figs. 31 and 35. Kellicott. 



ccel. This broad space is the archenteron which opens to the exterior through 

 the annular groove which was described on surface view, the opening being 

 the blastopore. The yolk cells which are inside the ring can here be seen to 

 fill the blastopore like a plug; collectively they are called the yolk plug. It 

 should also be noted that the yolk cells form an elevation in the floor of the 

 blastoccel on the side opposite the invagination. As a matter of fact the 

 elevation occurs all the way round the blastoccel as does also the cleft between 

 the elevation and the smaller cells. 



Invagination is probably not as important a factor here as it seems to be 

 although it plays a part; it certainly is not as important as in Amphioxus. 

 It must be remembered that the cells of the germ ring are multiplying rapidly 

 when the invagination groove appears. The rapid proliferation continues 

 during the processes thus far observed and many cells migrate inward around 

 the lip of the blastopore. This perhaps is comparable with a similar series 

 of rapid divisions and migration in the germ ring in Amphioxus. Conse- 



