FERTILIZATION OF THE OVUM. 



458 



Fig. 138. 



tion ; and he found that when the interval of time between the one 

 application and the other was only one or two seconds, only the early 

 stages of the process took place, and no embryo was produced ; when 

 an interval of five seconds was allowed, very few embryos were produced 

 from a large number of ova ; but when the interval was fifteen seconds 

 or more, the proportion of embryos produced was much greater. Thus 

 it seems obvious that time is an important element in the fertilizing 

 process ; and that fertilization may be incompletely effected, for want 

 of a sufficient penetration of the product of the diffluence of the Sperma- 

 tozoa. How this product acts upon the contents of the ovum, however, 

 and whether one or many of the cells set free by the rupture or solution 

 of the germinal vesicle are fertilized by it, have not yet been ascertained. 

 805. The first change which can be observed to be consequent upon 

 fecundation in the Mammalian ovum is the " segmentation" of the yolk; 

 the entire- mass of which, though previously 

 compact and uniform, resolves itself, first 

 into two, then into four, then into eight seg- 

 ments (Fig. 138) ; each segment containing a 

 transparent vesicle, which may be surmised 

 to be a descendant of the original germ-cell. 

 By a continuance of the same process, the ; 

 whole cavity of the vitelline sac, or zona pel-/ 

 lucida, becomes occupied by spherical parti-J 

 cles of yolk (each containing a pellucid p&r-yM 

 tide), the aggregation of which gives it a 

 mulberry-like appearance ; and by its fur- 

 ther continuance, the component cells becom- 

 ing more and more minute, the mass comes 

 to present a uniform finely-granular aspect. 

 At this stage it does not appear that the se- 

 veral segments of the yolk have a distinct 

 enveloping membrane ; but an envelope is 

 now formed around each of them, converting 

 it into a cell, of which the included particle 

 forms the nucleus. This happens first to the 

 peripheral portions of the mass ; and as its 

 cells are fully developed, they arrange them- 

 selves at the surface of the yolk into a kind 

 of membrane ; at the same time assuming a 

 pentagonal or hexagonal shape from mutual 

 pressure, so as to resemble pavement epithe- 

 lium (Fig. 139). As the globular masses of 

 the interior are gradually converted into cells, 



they also paSS tO the Surface and accumulate tion of the vitellus of the Mammalian 

 , i r , i , i . ! f. , ! Ovum : A, its first division into two 



there; thus increasing the thickness of the halves: B, subdivision of each half into 

 envelope already formed by the more super- ^^S^SSS^^. 

 ficial layer of cells, while the central part of 



the yolk remains, filled only with a clear fluid, which seems to be the 

 produce of the liquefaction of some of the interior spherules. By this 

 process, the external part of the yolk is converted into a kind of secon- 



^ Q stages in the segmenta- 



