CLEAVAGE OF THE YELK. 741 



a layer of transparent albuminous or glutinous substance, 

 which forms upon the exterior of the zona pellucida. It 

 is at first exceedingly fine, and, owing to this, and 

 to its transparency, is not easily ~ * 



recognized : but at the lower part 

 of the Fallopian tube it acquires 

 considerable thickness. 



About this time, that it is to say, 

 during its passage through the Fal- 

 lopian tube, a very remarkable 

 change takes place in the interior 

 of the ovum. The whole yelk be- 

 comes constricted in the middle, 

 and surrounded by a furrow, which, 

 gradually deepening, at length cuts 

 the yelk in half, while the same 

 process begins almost immediately 

 in each half of the yelk, and cuts 

 it also in two. The same process 

 is repeated in each of the quarters, 

 and so on, until at last by continual 

 cleavings the whole yelk is changed 

 into a mulberry-like mass of small 

 and more or less rounded bodies, 

 sometimes called " vitelline spheres," 

 the whole still enclosed by the zona 

 pellucida or vitelline membrane (fig. 

 208). Each of these little spherules contains a transparent 

 vesicle, like an oil-globule, which is seen with difficulty, 

 on account of its being enveloped by the yelk-granules 

 which adhere closely to its surface. 



The cause of this singular subdivision of the yelk is 



* Fig. 208. Diagrams of the various stages of cleavage of the yelk 

 (after Dalton). 



