THE FCETUS. 



891 



of fecundation itself. After it has taken place, the germinal vesicle is not 

 perceived ; has it disappeared, or changed its character if it does yet exist ? 

 This question cannot be answered; all that is known is that it has ceased 

 to be visible. Then the contraction of the vitellus begins ; it leaves its 

 enveloping membrane, becomes depressed in a circular manner, and ends by 

 forming two independent masses, each provided with a nucleus. These two 

 globes of segmentation (or cleavage masses) for such is the name given to 



Fig. 417. 

 A. B. C. 



PROGRESSIVE STAGES IN THE SEGMENTATION OF THE YOLK OF THE MAMMALIAN OVUM. 



A, Its first division into halves, with the spermatozoa around its circumference ; 

 B, Subdivision of each half into two ; c, Further subdivision producing numerous 

 segments. 



them are, in their turn, divided into other two globes, the number now being 

 four. These four are once more divided, and these again subdivided, until 

 the whole mass of the vitellus is completely transformed into globules 

 (or embryo-cells), composed of a nucleus covered by a thin membrane. 



A. 



Fig. 418. 



13. 



LATER STAGE IN THE SEGMENTATION PROCESS. 



A, Mulberry mass formed by the minute subdivision of the vitelline spheres; B, A 

 further increase has brought its surface into contact with the vitelline membrane, 

 against which the spherules are flattened. 



This is the segmentation of the ovulum : an operation that is always 

 preceded by the retraction of the vitellus, and the appearance of one or tv/o 

 small globules about the point where it commences to be depressed ; these 

 Robin has designated polar globules. 



