Chap. VI.] GENERAL EMBRYOLOGY. 97 



in the rabbit's ovum previous to the rupture of the follicle. As 

 it passes down the Fallopian tube the egg becomes invested with 

 an albuminous envelope. 



The Establishment of the Germinal Layers. (1.) We have 

 seen that in the frog there is at the close of segmentation a blasto- 

 sphere containing a comparatively small segmentation cavity 

 placed excentrically, roofed over with several layers of smaller 

 cells, and resting on a considerable mass of yolk-containing cells 

 (Fig. 34, i.). So far there is no differentiation into epiblast and 

 hypoblast. The large amount of food-yolk and the mass of 

 lower cells prevents invagination from taking place in so simple 

 a manner as in our illustrative case. Still invagination does take 

 place. The small roofing cells gradually creep over the rest of the 

 sphere, by the conversion at the edges of the roof of large yolk- 

 containing cells into small roofing cells. But having reached the 

 equatorial line, they grow in over a small arc, forming a sort of 

 lip. This marks the hinder end of the future embryo. The lip 

 extends further and further inwards and upwards so as to run 

 parallel, or rather concentric, to the dorsal surface. This exten- 

 sion of the lip is due partly to a true ingrowth of the external 

 roofing cells, but perhaps partly also to a differentiation of the 

 yolk-containing cells along its line. As it extends inwards and 

 upwards a slit-like cavity opens beneath it (Fig. 35, i.). Mean- 

 while the smaller roofing cells creep further and further round 

 the rest of the sphere, until only a small patch of the larger 

 yolk-containing cells remains uncovered. As the slit-like mesen- 

 teron enlarges, the original segmentation cavity is gradually 

 obliterated. Eventually the plug of yolk-containing cells be- 

 neath the inturned lip disappears, and the mesenteron com- 

 municates with the exterior by a small circular opening. 



Here, then, we have the gradual formation of an external layer 

 of cell ^ which turns inwards so as to line the roof of a cavity 

 which eventually opens to the exterior by a posterior orifice. 

 The external layer is the epiblast ; the internal layer continuous 

 with it at the lip is the hypoblast ; the slit-like cavity is the 

 mesenteron; and the posterior orifice is the blastopore. The 

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