Chap. VI.] GENERAL EMBRYOLOGY. 101 



upper layer of cells, which constitute the mesoblastic layer. In 

 this region, the cells in the median line (which longer retain their 

 primitive connection with the hypoblast, Fig. 35, v.) become 

 concentrated into the notochord (ch\ while the two mesoblastic 

 plates, one on each side, become more and more completely 

 differentiated. The upper parts of these plates give rise subse- 

 quently to a vertebral plate segmenting into mesoblastic somites, 

 the lower parts, or lateral plates, being cleft into two layers, 

 enclosing the body-cavity. 1 



(3.) We have seen that in the rabbit, at the close of segmen- 

 tation, an outer layer of clear cells surrounds, except at one 

 spot, the pseudo-blastopore, an inner mass of granular cells. It 

 is possible, as above suggested, that the slipping inwards of the 

 inner cells is analogous to the process of invagination. But if 

 so, this process is a secondary one, and the pseudo-blastopore is 

 probably not homologous with the blast opore. 



The outer cells ere long grow over the pseudo-blastopore, and 

 completely enclose the inner cells. A fissure then makes its 

 appearance between the inner cells and the outer cells, on that 

 side of the ovum which is furthest from the pseudo-blastopore. 

 The cells separate more and more, until a relatively large 

 cavity is formed. And this cavity increases more and more, 

 until the inner cells form merely a lens-shaped mass attached 

 to the inner side of the outer cells (Fig 35, ii.). Thus, by 

 an internal separation of the cells, is formed a blasto- 

 dermic vesicle The greater part of its walls consists of a 

 single layer of flattened outer cells. It rapidly enlarges, the 

 cavity being filled with an albuminous fluid ; and as it continues 

 to enlarge, the lens-like mass of inner cells spreads out and 

 becomes flattened, so that there is formed a central area where 

 the cells are two rows thick, and a peripheral area where they 

 are one row thick In the central, or embryonic area, the lower 

 layer of inner cells forms the flattened hypoblast, while the 

 upper layer of inner cells fuses with the layer of outer cells to 



1 The cleavage of the mesoblast at first extends into the vertebral plates, but 

 is here, for the most part, obliterated, though even here it may subsequently 

 reappear. 



