THE EARLY DEVELOPMENT OF THE FROG 99 



the frog egg, and its accumulation chiefly in one region. The 

 effect of the yolk in modifying the course of development be- 

 comes more marked in the immediately succeeding phases of 

 development, namely gastrulation and notogenesis; these 

 processes are not at all as simple and diagrammatic as in 

 Amphioxus. 



5. Gastrulation and .Notogenesis 



The processes of gastrulation proper and of notogenesis 

 overlap to a more considerable extent in the frog, than in 

 Amphioxus, and are conveniently described together. Gas- 

 trulation results directly from a continuation of the downward 

 extension of the germ ring, together with the consequent 

 elevation of the yolk cells, which were such important features 

 in the development of the late blastula. 



The first external indication of gastrulation is the appearance 

 f a slight irregular groove, approximately horizontal, lying 

 across the sagittal plane on the posterior side of the egg, just 

 at the lower margin of the germ ring, i.e., just below the 

 equator (Figs. 32, 35, A). Thus located, the groove lies just 

 between the animal cells and the yolk cells, and therefore 

 comes to be lined by both kinds of cells on its opposite faces. 

 From subsequent development we know that the formation 

 of this groove is the beginning of invagination, the groove 

 itself the beginning of the archenteron, its upper margin the 

 rim of the blastopore, and the cells lining it above and below, 



of the whole gastrula, as well as the phenomena of gastrulation proper. A. Com- 

 mencement of gastrulation; earliest appearance of the dorsal lip of the blasto- 

 pore. 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 indentation of the wall of the segmen- 

 tation 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 estab- 

 lished. G. Gastrulation completed, a, Archenteron; 6, 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 blastoccel. 



