THE GERMINAL LAYERS. 237 



becomes oval, and then pyriform in shape, the broader end 

 corresponding to the anterior end of the embryo. 



The two layers of the blastoderm grow independently. The 

 epiblast, after it has become free from the lower layer, extends 

 slightly beyond this, so that its margin rests directly on the 

 yolk ; its further spreading is effected mainly by division of the 

 already formed cells, stimulated, no doubt, by absorption of nutri- 

 ment from the yolk on which they are lying. The lower-layer 

 cells, after separation from the epiblast, become directly con- 

 tinuous at their margin with the yolk, forming a thickened rim, 

 spoken of as the germinal wall : the extension of the lower- 

 layer cells is effected principally by the addition of new cells cut 

 out from the yolk, but partly also by division of the already 

 formed cells, as in the epiblast. 



3. The Hypoblast. 



A few hours after the commencement of incubation, the 

 lower-layer cells undergo important changes, by which the hypo- 

 blast and mesoblast become established. 



In the area pellucida, the majority of the lower-layer cells 

 become flattened horizontally, and unite at their edges so as to 

 form a continuous cellular membrane, the hypoblast ; a few 

 isolated lower-layer cells are left between the epiblast and the 

 hypoblast, which take part, as will be noticed immediately, in 

 the formation of the mesoblast. 



In the area opaca, or marginal part of the blastoderm, the 

 differentiation of the hypoblast as a distinct cellular membrane 

 occurs somewhat later ; and the hypoblast cells of this region, 

 which are large, and cubical or slightly columnar in shape, differ 

 markedly from the thin, pavement, hypoblast cells of the area 

 pellucida. 



1. The Primitive Streak. 



At the posterior border of the blastoderm, as noticed above, 

 the fusion of the epiblast and the lower-layer cells persists 

 longer than it does round the rest of the blastodermic rim ; 

 and in the egg, at the time of laying, a crescentic opacity is 

 visible at the posterior edge of the blastoderm, marking this 

 line of fusion. 



As the blastoderm grows, during the earlier hours of incu- 



