102 FORMATION OF THE LAYERS. 



equatorial line which marks the junction between the epiblastic cells 

 and the yolk-cells (fig. 70, x). 



The inflected cells become continuous with the adjoining cells ; 

 and the region where the inflection is formed constitutes a kind of 

 lip, below which a slit-like cavity is soon established. This lip is 

 equivalent to the embryonic rim of the Elasmobianch blastoderm, and 

 the cavity beneath it is the rudiment of the mesenteron. 



The mesenteron now rapidly extends by the invagination of the 

 cells on its dorsal side. These cells grow inwards towards the segmen- 

 tation cavity as a layer of cells several rows deep. At its inner end, 

 this layer is continuous with the yolk-cells ; and is divided into two 

 strata {fig. 71 A), viz. (Ij a stratum of several rows of cells adjoining 

 the epiblast, which becomes the mesoblast {m), and (2) a stratum of a 

 single row of more columnar cells lining the cavity of the mesenteron, 

 which forms the hypoblast {hy). The growth inwards of the dorsal 

 wall of the mesenteron is no doubt in part a true invagination, but 

 it seems probable that it is also due in a large measure to an actual 

 differentiation of yolk-cells along the line of growth. The mesenteron 

 is at first a simple slit between the yolk and the hypoblast (fig. 71 A), 

 but as the involution of the hypoblast and mesoblast extends further 

 inwards, this slit enlarges, especially at its inner end, into a con- 

 siderable cavity ; the blind end of which is separated by a narrow 

 layer of yolk-cells from the segmentati(jn-cavity (fig. 71 B). 



In the course of the involution, the segmentation-cavity becomes 

 gradually pushed to one side and finally obliterated. Before oblitera- 

 tion, it appears in some forms (Pelobates fuscus) to become completely 

 enclosed in the yolk-cells. 



While the invagination to form the mesenteron takes place as 

 above described, the enclosure of the yolk has been rapidly proceed- 

 ing. It is effected by the epiblast growing over the yolk at all points 

 of its circumference. The nature of the growth is how^ever very 

 different at the embryonic rim and elsewhere. At the embryonic rim 

 it takes place by the simple growth of the rim, so that the point x in 

 figs. 70 and 71 is carried further and further over the surface of the 

 yolk. Elsewhere the epiblast at first extends over the yolk as in a 

 typical epibolic gastrula, without being inflected to form a definite 

 lip. While a considerable patch of yolk is still left uncovered, the 

 whole of the edge of the epiblast becomes however inflected, as at 

 the embryonic rim (fig. 71 A); and a circular blastopore is established, 

 round the whole edge of which the epiblast and intermediate cells 

 are continuous. 



From the ventral lip of the blastopore the mesoblast (fig. 71, m), 

 derived from the small intermediate cells, grows inwards till it comes 

 to the segmentation-cavity ; the growth being not so much due to an 

 actual invagination of cells at the lip of the blastopore, as to a 

 differentiation of yolk-cells in situ. Shortly after the stage repre- 

 sented in fig. 71 B, the plug of yolk, which fills up the opening of 

 the blastopore, disappears, and the mesenteron communicates freely 



