FORMATION OF THE BLASTODERM. 



17 



sixteen clear cells and only eight darker cells, for the same reason. Further, it is 

 found that as the segmentation proceeds the clearer cells occupy the superficial part 

 of the ovuin and almost entirely enclose the granular cells which fill the interior 

 (fig, 14, a). 



The ovum next undergoes a rapid increase in size owing to the segregation of 

 fluid between the clear superficial layer of cells and the enclosed granular segments, 

 which thus become separated from one another except at one part (fig. 14, &). At the 

 same time the superficial cells multiply, and, becoming flattened out like a pavement 

 epithelium, form a membrane enclosing the contained fluid. The ovum is now a 

 thin-walled transparent sac, occupied by fluid and enclosed by two membranes, one 



ent. 



Fig. 14. SECTIONS OF THE OVUM OF THE RABBIT 



DURING THE LATER STAGES OF SEGMENTATION, 

 SHOWING THE FORMATION OF THE BLASTODER- 



MIC VESICLE. (E. v. Beneden.) 



a, Section showing the enclosure of darker cells, 

 cnt, by clearer cells, cct ; b, more advanced stage 

 in which fluid is beginning to accumulate be- 

 tween the inner and outer cells, the former com- 

 pletely enclosed ; c, the fluid has much increased, 

 so that a large space separates inner from outer 

 cells except at one part ; d, blastoderm ic vesicle, its 

 wall formed of a layer of flattened cells, with a 

 patch of dark granular cells adhering to it at one 

 part ; z.p., zona pellucida. 



being the thinned-out zona pellucida and 



the other the epithelial membrane just 



mentioned. Adherent to one part of the 



inner surface of this membrane is the 



little mass of dark granular cells which formerly occupied the whole interior of the 



mulberry mass, and these cells give to the part of the ovum where they occur a 



darker appearance, when it is viewed by transmitted light. At this stage of 



development the ovum has been termed the Uastodermic vesicle (fig. 14, c, d), 



although the actual blastoderm is not yet formed. 



Formation of the blastoderm. Soon the granular cells are found to be no 

 longer accumulated into a small mass but to be spreading out in the form of a 

 lenticular patch over the inner surface of the vesicle. As this extension proceeds 

 the innermost cells separate off as a distinct layer, the separation starting from 

 the centre and progressing outwards. 



A section through the middle of the ovum now shows three layers (fig. 15) : an 

 outer, which is the epithelial membrane of the blastodermic vesicle (Rauber's layer) ; 

 an inner, which may be termed the primitive entoderm, from the fact that it becomes 

 the innermost layer of the blastoderm, and an ill-defined middle stratum of somewhat 



VOL. i. c 



