86 FORMATION OF THE LAYERS. 



finally (about the 2ooth hour) obliterated. The roof of the 

 mesenteron is formed by the continued invagination of small 

 cells, and its floor is composed of large yolk-cells. The wide 

 external opening is arched over dorsally by a somewhat promi- 

 nent lip the homologue of the embryonic rim. The opening 

 persists till nearly the time of hatching ; but eventually becomes 

 closed, and is not converted into the permanent anus. On the 

 formation of the mesenteron the hypoblast is composed of two 

 groups of cells, (i) the yolk-cells, and (2) the cells forming the 

 roof of the mesenteron. 



While the above changes are taking place, the small cells, or 

 as they may now be called the epiblast cells, gradually spread 

 over the large yolk-cells, as in normal types of epibolic invagi- 

 nation. The growth over the yolk-cells is not symmetrical, but 

 is most rapid in the meridian opposite the opening of the 

 alimentary cavity, so that the latter is left in a bay (cf. Elasmo- 

 branchii, p. 63). The epibolic invagination takes place as in 

 Molluscs and many other forms, not simply by the division of 

 pre-existing epiblast cells, but by the formation of fresh epiblast 

 cells from the yolk-cells (fig. 37) ; and till after the complete 

 enclosure of the yolk-cells there is never present a sharp line of 

 demarcation between the two groups of cells. By the time that 

 the segmentation cavity is obliterated the whole yolk is en- 

 closed by the epiblast. The yolk-cells adjoining the opening 

 of the mesenteron are the latest to be covered in, and on their 

 enclosure this opening constitutes the whole of the blastopore. 

 The epiblast is composed of a single row of columnar cells. 



Mesoblast and notochord. During the above changes the 

 mesoblast becomes established. It arises, as in Elasmobranchs, 

 in the form of two plates derived from the primitive hypoblast. 

 During the invagination to form the mesenteron some of the 

 hypoblast cells on each side of the invaginated layer become 

 smaller, and marked off as two imperfect plates (fig. 38, ms). 

 It is difficult to say whether these plates are entirely derived 

 from invaginated cells, or are in part directly formed from the 

 pre-existing yolk-cells, but I am inclined to adopt the latter 

 view ; the ventral extension of the mesoblast plates undoubtedly 

 takes place at the expense of the yolk-cells. The mesoblast 

 plates soon become more definite, and form (fig. 39, ins] well- 



