114 DEVELOPMENT OF THE FROG 



mid-dorsal line a rod-like thickening of the hypoblast is formed 

 at a very early stage. This is the notochord (Figs. 24 to 26, N), 

 which 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 hinder end, where it remains for some time in continuity 

 with both hypoblast and epiblast at the lip of the blastopore. 



The Mesoblast. Between the epiblast and hypoblast a third 

 or intermediate layer of cells, the mesoblast, is soon established. 



It is formed by differentiation of the surface hypoblast and 

 yolk-cells as a separate layer, 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. (C/. Fig. 2fi.) In many 

 specimens the mesoblast cells are from the first arranged in 

 two layers ; and in some cases the mesoblast may be distinctly 

 seen to arise as a pair of flattened hollow outgrowths from the 

 hypoblast, a little way in front of the blastopore, and at the 

 sides of the notochord : these outgrowths spread in all directions, 

 and their cavities become the body cavity or coelom. Such a 

 mode of origin of the mesoblast is known to obtain in some 

 lower vertebrates, but it is uncertain as yet how far it is general 

 in the case of the frog. 



Growth of the hypoblast. The hypoblast is formed in the 

 iirst instance from the epiblast by invagination at the lip of the 

 blastopore : its after growth is effected, however, mainly at the 

 expense of the yolk-cells, with which it is in contact. By 

 growth at its margin it gradually creeps round until it extends 

 all round the embryo, the yolk-cells forming part of its 

 ventral wall. 



The two forms of hj^oblast may be named, according to 

 their modes of origin, invaginate hypoblast and yolk hypoblast 

 respectively. They may be easily distinguished in the early 

 stages of development by the fact that the invaginate hypoblast, 



