THE OVUM. 



1153 



enclose the lower cells, so that by the ninth or tenth division there is an external 

 layer of pale cells enclosing a mass of slightly smaller, more opaque cells, which, 

 in consequence of their diminished rate of cleavage, are fewer in number (Fig. 



Blastodermic 



Inner mass of cells. 



\Hjipoblnxt. 



;Raubei''s layer. 



FIG. 693. Blastodermic vesicle of Vespertilio murinus. (After Van .Beneden.) (Reduced from a drawing in 

 the Anatumischer Anzeiger, xvi. Band, Sept. 8, 1899.) 



692). Fluid collects between the two sets of cells, except at one part, termed 

 the embryonal pole, so that a vesicle, the blastodermic vesicle, is formed. This 

 vesicle consists of an outer layer of cells, termed Rauber's layer, derived from the 



Commencing amniotic cariti/. 



Rauber's layer. 



Hypobltut. 



Epiblast. 



FIG. 694. Section through embryonic area of Vespertilio murinus. (After Van Beneden.) (Reduced from a 

 drawing in the Anatmnischer Anzeiger, xvi. Band, Sept. 8. 1899.) 



subdivision of the primary upper cell, enclosing at the embryonal area an inner 

 mass of cells (Fig. 693) resulting from the cleavage of the primary lower cell. 

 Rauber's layer takes no share in the formation of the embryo proper, which is 

 entirely developed from the inner mass of cells. The deepest cells of this mass 



Amniotic carity. 



Rauber 



's layer j 



Plasmodioblast 

 and cytoblast. 

 Epiblast. 



Epiblast. Hypoblast. 



Fit;. 695. Section through embryonic area of Vespertilio murinus (after Van Beneden), to show the forma- 

 tion of the amniotic cavity. (Reduced from a drawing in the Anastomischer Anzeiger, xvi. Band, .Sept. 8, 1899.) 



become differentiated as a layer of flattened cells, termed the liypoblast, which 

 spreads outward beneath Rauber's layer. The latter, by subdivision of the cells 

 of its upper hemisphere, is differentiated into two strata, the outer of which 

 becomes rapidly thickened and forms a plasrnodioblast (i. e., a mass of protoplasm 

 containing numerous nuclei, but not subdivided into individual cells by means of 

 73 



