THE GEEMIXAL LAYERS. 237 



b ecomes oval, and then PYriforin in shap e, t he broader en d 

 corr esponding to the anterior end nf t.tie embryo . ' 



T he two laye rs 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. Tbg ^ lower-layer 

 cells, afte r separation from the epiblast , become directly con- 

 tinuous at their ma rgin 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 jDartly also by division of the already 

 formed cells, as in the epiblast. 



3. The Hypoblas t. 



A few hours after the commencement of incubation, the 

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

 blast and mesoblast become established. 



I n the area pellucid a, th e majority of the lower-layer cell s 

 bec ome flattened horizontall y, and unite at their edges so as to 

 for m a continuous cellular memb rane, the h^oblast ; 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 th e area opaca. or marginal part of the blastoderm, th e 

 differentiation of the hypoblast as a di stinct cellular membra ne 

 occ urs som ewhat later ; and the hypoblast ceils 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. 



4. The Primitive Strea k. 



At th g posterior borde r of the blastoderm, as noticed above, 

 the fus ion of the epiblast and t he lower-layer cells p ersists 

 lo nger th an it does r ound the rest of the blastodermic rimj 

 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- 



