558 



THE EMBRYOLOGY OF THE FROG 



As the blastopore closes, it carries mesodermal cells toward the mid- 

 line (Fig. 324), where they form a broad median band extending forward 

 from the dorsal lip of the blastopore. These cells then multiply, and as 

 the dorsal lip extends downward, an axial thickening is formed. At this 

 same time, the archenteron extends and carries yolk-cells outward toward 

 the animal pole, so that the extent of the mesoderm is almost as great as 

 that of the entoderm. 



Fig. 324. 



Frontal and transverse sections through gastrulas of the frog (R. temporaries) 

 of various ages. A. Frontal section through gastrula of same age as Fig 323. 

 C. B. Frontal section through gastrula of same age as Fig. 323, D. C. Frontal 

 section through gastrula slightly older than Fig. 323, F. D. Frontal section 

 through gastrula of same age as Fig. 32, G. E. Transverse section through gastrula 

 slightly older than Fig 323, D. F. Transverse section through gastrula slightly 

 older than Fig 323, G. a, Archenteron ; b, blastopore ; c, notochord ; ge, gut 

 endoderm ; m, peristomial mesoderm ; np, neural plate ; s, segmentation cavity or 

 blastocoele. (After Brachet.) 



Then a rearrangement of cells takes place, so that an irregular de- 

 lamination commences in the dorsal lateral regions on each side of the 

 thickened axial mass, and extends from there anteriorly and laterally 

 around the sides of the archenteron. Thus, a thick layer of mesoderm is 

 formed between the entodermal lining of the archenteron and the outer 

 ectoderm (Fig. 324, m). 



At the lower pole, that is, toward the place where the yolk-plug is 

 being drawn into the interior of the egg (Fig. 324, b), the lower surface 

 of the yolk is also delaminated so that a circular margin of the mesoderm 

 is formed there. It is from this layer of mesoderm that cells and groups 

 of cells bud off and pass toward the lower pole (it is to be remembered 

 that these cells and groups of cells begin their growth in the lower pole 



