DE VEL OPMENT. 499 



At the close of segmentation the blastoderm is a lens- 

 shaped disc with two strata of cells. It is thicker at one 

 end — where the embryo begins to be formed. Towards the 

 other end, between the blastoderm and the yolk, lies a 

 segmentation cavity (Fig. 213, sg.c). 



At the embryonic end the outer layer or epiblast under- 

 goes a slight invagination (Fig. 213, x.), beginning to form 

 the roof of the future gut (g.) ; in other words, establishing 

 the hypoblast. This inflected arc of the blastoderm corre- 

 sponds to the blastopore or mouth of the gastrula, which is 

 much disguised by the presence of a large quantity of yolk. 

 As the invagination proceeds, the segmentation cavity is 

 obliterated. The floor of the gut is formed by infolding of 

 the lateral walls. 



Along the mid-dorsal line of the epiblast a medullary 

 groove appears — the beginning of the central nervous 

 system. Its sides afterwards arch towards one another, and 

 meet to form a medullary canal (Fig. 213, n.c). A posterior 

 communication between this dorsal nervous tube above and 

 the ventral alimentary tube persists for some time as the 

 neurenteric canal (213, n.e.c). 



The mesoblast arises as two lateral plates, one on each 

 side of the medullary groove. The plates seem to arise as 

 a pair of solid outgrowths from the wall of the gut. They 

 are afterwards divided into segments. Between the meso- 

 blast plates, along the mid-dorsal line of the gut, the 

 notochord is established (Fig. 213, «.). 



Besides the internal establishment and differentiation of 

 layers, there are two important processes, — (a) the growth of 

 the blastoderm around the yolk, (6) the folding off of the 

 embryo from the yolk. The result of the two processes 

 is that the yolk is enclosed in a yolk-sac, with which the 

 embryo is finally connected only by a thin stalk — the 

 umbilical cord. 



The history of the yolk is briefly as follows : — It is accumulated 

 by the ovum from neighbouring cells, and from the vascular fluid ; it 

 is partly prepared for absorption by the merocytes or yolk-nuclei ; it is 

 at first absorbed by the blood vessels of the yolk-sac ; at a later stage, 

 absorption by blood vessels becomes less and less important, and the 

 yolk passes inside the embryo and into the gut, where it is digested ; the 

 yolk-sac, empty of all but merocytes, degenerates, shrivels, and 

 disappears. 



