THE EARLY DEVELOPMENT OF THE FROG 107 



oxus, we have seen, imagination is the chief process, while in 

 the frog this is less important, and the endoderm is more largely 

 the result of delamination or of a simple rearrangement of cells 

 forming different parts of the wall of the blastula. Among the 

 higher Chordates invagination may be entirely lacking and gas- 

 trulation may be entirely accomplished by other methods (in- 

 volution, epiboly, delamination). As a matter of fact, even in 

 t he frog, invagination is concerned less with the formation of the 

 inner layer than with the establishment of the notochord and the 

 formation of the rudiment which gives rise in part to the meso- 

 derm. It becomes necessary, therefore, to distinguish sharply 

 between gastrulation and notogenesis. In the frog the strictly 

 two-layered condition exists for a very brief period only, for the 

 earliest phases of notogenesis, namely the formation of the meso- 

 derm and chorda, occur quite precociously. The early forma- 

 tion of these structures may more conveniently be described 

 together, and for the first stages we must return to the early 

 gastrula. 



6. The Mesoderm 



In order to understand the origin of the mesoderm we must 

 examine the early gastrula at the time the gastrular groove and 

 cleavage extend down toward the incipient blastopore. Here 

 the inner region of the germ ring and the yolk cells lining the 

 blustocu'l are continuous, and it is here that we find those cells 

 which are later to form the mesoderm, and although distin- 

 guishable at this time, they are not definitely delimited within 

 this zone which is transitional between the ectoderm and endo- 

 derm. On one side these cells are continuous with ectoderm 

 cells, on the other with endoderm or yolk cells (Figs. 32, A; 33, A). 

 As the lips of the blastopore extend laterally this mesoderm 

 rudiment forms pari passu, and when the blastopore rim becomes 

 circular the mesoderm rudiment can be distinguished in the 

 ventral lip (Figs. 32, F; 33, C). We may say then that the rudi- 

 ment of the mesoderm appears first as a ring of cells just within 

 the margin of the blastopore. But by the time the ring is com- 



