The whole process of segmentation is completed in the rabbit about seventy hours after impregnation. At its close the epiblast cells, as they may now be called, are clear, and have an irregularly cubical form; while the hypoblast cells are polygonal and granular, and somewhat larger than the epiblast cells.
The opening in the epiblastic layer where the hypoblast cells are exposed on the surface may for convenience be called with Van Beneden the blastopore, though it is highly improbable that it in any way corresponds with the blastopore of other vertebrate ova[76].
Fig. 134. Optical sections of a Rabbit’s ovum at two stages closely following upon the segmentation. (After E. van Beneden.)
ep. epiblast; hy. primary hypoblast; bp. Van Beneden’s blastopore. The shading of the epiblast and hypoblast is diagrammatic.
After its segmentation the ovum passes into the uterus. The epiblast cells soon grow over the blastopore and thus form a complete superficial layer. A series of changes next take place which result in the formation of what has been called the blastodermic vesicle. To Ed. van Beneden we owe the fullest account of these changes; to Hensen and Kölliker however we are also indebted for valuable observations, especially on the later stages in the development of this vesicle.
The succeeding changes commence with the appearance of a narrow cavity between the epiblast and hypoblast, which extends so as completely to separate these two layers except in the region adjoining the original site of the blastopore ([fig. 134] B)[77]. The cavity so formed rapidly enlarges, and with it the ovum also; which soon takes the form of a thin-walled vesicle with a large central cavity. This vesicle is the blastodermic vesicle. The greater part of its walls are formed of a single row of flattened epiblast cells; while the hypoblast cells form a small lens-shaped mass attached to the inner side of the epiblast cells ([fig. 135]).
Fig. 135. Rabbit’s ovum between 70-90 hours after impregnation. (After E. van Beneden.)
bv. cavity of blastodermic vesicle (yolk-sack); ep. epiblast; hy. primitive hypoblast; Zp. mucous envelope (zona pellucida).