1006 



EMBRYOLOGY. 



Fi£j. 542. 



fecundation ; and although we have not to treat of fecundation itself, yet it ia 

 well to bring forward the essential fact resulting from modern investigations 

 on this point, showing that this great act consists in the fusion of two germs 

 — the fpniale pronucleus, arising from the division of 

 the germinal vesicle ; and the male pronucleus, due to 

 the transformation and migration of the spermatozoid. 

 These two elements, by their fusion, originate the yolk- 

 nucleus which, by its double origin, contains the material 

 elements of hereditary continuity. 



The division of the yolk-nucleus begins and deter- 

 mines the segmentation of the yolk — equivalent to cell- 

 proliferation. 



Before becoming segmented, the yolk separates from 

 its enveloping membrane, retracts, and leaves around it 

 a space occupied by a transparent fluid. 



The segmentation of the yolk is total, the entire 

 mass undergoing proliferation, and the first effect of 

 this is the production of two cells resulting from the 

 division of the yolk. These two primary cells, or 

 segmentation globes, in their turn divide and give rise 

 to two generations which have different destinies to 

 fulfil. This specialization is already naturally contained 

 in the two primary globes of segmentation, and is 

 marked by particular characters. The two globes are, 

 in fact, unequal in size. The largest — the ectodermic 

 globe — is transparent ; the second — the endodermic globe 

 — is charged with granules which are readily stained by osmic acid. 



The proliferation of the ectodermic and endodermic globes continues, and in 

 Buch a manner that the segmentation globes issuing from the first proceed towards 



A GRAAFIAN VESICLE, FROM 

 A WOMAN THIRTV-TWO 

 YEARS OF AGE. 



In this is seen the epithelial 

 covering formed of poly- 

 gonal cells, and an ovum 

 in which there is at a the 

 germinal vesicle with its 

 spot, and at b the embryo- 

 genous vesicle, much 

 smaller, and surrounded 

 by numerous granules 

 which are disseminated in 

 the vi tell us; c is the 

 zonula pellucida. 



OPTICAL SECTIONS OF THE OVUM OF A RABBIT IN TWO STAGES, SOON AFTER SEGMENTATION. 



*p, Epiblast, or ectoderm ; hy, hypoblast, or entoderm ; hp, blastopore. The ectoderm and endoderm 



are ditlerently shaded. 



the periphery, and tend to envelop the mass of those which arise from the division 

 of the endodermic globes. Finally, in about seventy hours after fecundation, in 

 the Rabbit, the envelopment is complete. 



