EARLY DEVELOPMENT OF THE FROG. 61 



ventrally between the other layers, although it is not complete for some 

 time (Fig. 33). There is ample evidence here that this portion of the meso- 

 derm is a derivative of entoderm (yolk entoderm). The mesoderm that 

 develops along the crescentic groove and around the blastopore is often 

 called peristomal; that which arises elsewhere is known as gastral mesoderm. 

 v/^ The behavior of the mesoderm that is involved in the dorsal axial strand 

 above or anterior to the blastopore is rather complex because out of that 

 strand arises one of the early axial structures of the embryo, the notocord. 

 First a slight cleft between ectoderm and mesoderm gradually extends from 

 each side toward the mid-dorsal line, but 

 just before reaching the line abruptly 



turns ventrally. This cleft as it bends :^^^5^^S^^^- ec 

 ventrally leaves a group of cells in the 

 axial line which is still continuous with 

 ectoderm above and entoderm below. 

 The axial group of cells is the rudiment 

 of the notocord. (Fig. 34.) Just above 

 or anterior to the blastopore, in the re- 

 gion where entoderm and mesoderm are , . 



FIG. 34. Portion of a transverse section 



still continuous at the lower lateral angles O f the larva of a frog (Rana 



c,i IT fusca). Hertwie. a, Archenteron; 



of the notocord rudiment, a pair of grooves c ind < cates en terocoel formation^ 



appear which are not particularly con- ec, ectoderm; en, entoderm; m, 



. ... . . mesoderm; , notocord; p, neural 



SplCUOUS but which seem to be evagma- plate; y, yolk entoderm. 



tions from the archenteron (Fig. 34). 



Farther forward the grooves are slightly more conspicuous, but still 

 farther forward disappear. It has been argued that these grooves are 

 homologous with the enteroccelic evaginations in Amphioxus where the 

 mesoderm arises by outgrowth from the entoderm. On the other hand it has 

 also been argued that the more primitive mode of mesodermal development 

 is represented in the frog and that the simplicity of origin is secondarily 

 acquired in Amphioxus. Whether pr not these grooves are enteroccelic 

 evaginations in the frog, soon after their appearance the notocord rudi- 

 ment becomes separated from entoderm below, from ectoderm above, and 

 lies in the axial line between what are now the paraxial masses of meso- 

 derm on the two sides (Fig. 33 ) The notocord therefore becomes an inde- 

 pendent structure except at its caudal end where it merges with all the 

 layers which in turn are merged with one another at the blastopore. A 

 similar fusion is present in Amphioxus (p. 45) ; and out of this mass of cells, 

 as development proceeds from before backward, the three germ layers and 

 the notocord are differentiated. 



During gastrulation a certain shift in position of the structure as a whole 



