100 



EMBRYOLOGY. 



While the Mammalian egg is gradually impelled through the 

 oviduct toward the uterus by the ciliary motion of the epithelium, it 

 becomes converted by the cleavage process into a spherical mass of 

 small cells (fig. 58 .4). Then there arises within it, by the secretion 

 of a fluid, a small fissure-like cleavage-cavity (fig. 58 ). The germ 

 has consequently entered upon the vesicular or blastula stage. The 

 wall of the blastula, or vesicula blastodermica, is composed of a 

 single layer of polygonal cells, arranged, as has been known since 

 BISCHOFF'S works, in mosaic, with the exception of a small region, 

 where the wall, as in the case of the Amphibian blastula, is thickened 

 by an accumulation of somewhat more granular and darker cells, 



Fig. 58. Optical sections of a Rabbit's egg in two stages immediately following cleavage, after 

 ED. v. BENEDEN. Copied from BALFOUR'S " Comparative Embryology." 



A, Solid cell-mass resulting from cleavage. 



B, Development of the blastula by the formation of a cleavage-cavity in the cell-mass. (According 



to VAS BENEDEN'S interpretation, ep is epiblast ; ky, hypoblast ; bp, blastopore.) 



which produce a knob-like elevation that projects far into the 

 cleavage-cavity. 



A peculiarity preeminently characteristic of the further develop- 

 ment of Mammals is that here, as in no other Vertebrate, the 

 blastula increases enormously in size (fig. 59), by the accumulation 

 of fluid which contains much albumen and produces a granular 

 coagulum upon the addition of alcohol ; it soon acquires a diameter 

 of I/O mm. Of course, with these processes of growth the zona 

 pellucida is altered and distended into a thin membrane. A gela- 

 tinous layer (zp) already secreted by the oviduct envelops the 

 latter. 



In Rabbits' eggs which are a millimetre in diameter the wall of 

 the blastula has become very thin. The mosaic- like cells arranged 

 in a single layer have become very much flattened. Also the knob 



