FORMATION OF THE GERMINAL LAYERS. 123 



Fig. 26, MN, which gives rise to the greater part of the length 

 of the aUmentary canal of the embryo. The mesenteron still 

 communicates with the exterior through the blastopore, but 

 the permanent mouth and anus are not yet formed. 



The roof of the mesenteron is thin, and is formed of a 

 definite layer of cells spoken of as the hypoblast: the floor is 

 at first very thick, and is formed by the mass of the yolk-cells. 



During the process of formation of the mesenteron by in- 

 growth of the hypoblast cells the segmentation cavity gets 

 pushed out of place and ultimately obliterated. 



The Notochord. Along the roof of the mesenteron in the 

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

 at a very early stage. This is the notochord (Figs. 25 and 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 a,long 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 ujd 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. 27.) In many 

 specimens the mesobsast cells are from the first arranged in 

 two layers. 



Fate of the germinal layers. From one or other of the three 

 germinal layers, — epiblast, mesoblast, and hypoblast, — every 

 part of the embryo is formed. 



The epiblast, or outer layer, gives rise to the epidermis 

 covering the body generally, and to the various glandular and 

 other structures derived from the epidermis; to the nervous 



