116 DEVELOPMENT OF THE FROG 



reduced to a narrow chink and is almost stopped up by 

 the yolk plug. The permanent mouth and anus are not 

 yet formed. 



During the process of formation of the mesenteron, the 

 segmentation cavity gets pushed out of place and be- 

 comes reduced to a chink between the epiblast and hypo- 

 blast. (Fig. 25 A , SC.) 



The Mesoblapt. Between the epiblast and hypoblast a 

 third or intermediate layer of cells, the mesoblast, is soon 

 established. 



It is a separate layer formed by differentiation in part of 

 the subequatorial intermediate cells, and in part from the 

 yolk-cells lying immediately beneath the epiblast, but quite 

 distinct from it. It extends all round the embryo except 

 along the mid-dorsal line, where the space between the 

 epiblast and hypoblast is occupied by the notochord. It is, 

 for a time, incomplete in front, opposite the segmentation 

 cavity, but soon grows in from the sides so as to fill up the 

 deficiency. 



The cells of the mesoblast become early arranged in two 

 parallel layers or sheets, which separate slightly from each 

 other-, so as to leave between them a narrow space, which 

 later on becomes the body cavity or coelom. (Cf. Fig. 26.) 

 In many specimens the mesoblast cells are from the first 

 arranged in two layers. 



The Notochord. Along the roof of the mesenteron there 

 is a dorsal cell-mass where the roof of the mesenteron and 

 the lateral sheets of mesoblast are folded together. This is 

 the notochord, and is derived from both the intermediate 

 and yolk-cells. (Figs. 24 to 26, N.) It is formed at a very 

 early stage, and serves to slightly stiffen the back of the 

 embryo, and is for some time the only skeleton which it 

 possesses. 



It very early splits off from the roof of the mesenteron, 

 except at its liinder end, where it remains for some time in 

 continuity with both hypoblast and epiblast at the lip of the 

 blastopore. 



Growth of the hypoblast. The hypoblast is formed in the 

 first instance from the inner sheet of intermediate cells at 

 the lip of the blastopore : its aftergrowth is effected, how- 



