SEGMENTATION OF OVUM. 



103 



which takes no part in this process of division. Then, each of these two daughter 

 elements divides in like manner, and thus four nucleated elements are formed, 

 and so on, until at length a mulberry-like agglomeration of nucleated masses of 

 protoplasm results (Fig. To). These masses are sometimes termed segmentation 



The manner in which segmentation occurs is somewhat peculiar. The two 

 spheres resulting from the first cleavage are of unequal size. One, which for the 

 sake of distinction we will call the upper cell, is larger than the other, the lower 

 cell. And after they have divided three or four times the rate of cleavage in 

 the spheres derived from the upper segment becomes more rapid than in those 

 derived from the lower segment. In addition to this, the spheres derived from 

 the upper segment have a tendency to spread over and enclose those from the 

 lower segment : so that by about the ninth or tenth division there is an 

 external layer of spheres derived from the primary upper segment surround- 

 ing and almost enclosing a mass of spheres, which in consequence of their 

 diminished rate of cleavage are fewer in number and larger in size, derived from 

 the primary lower segment (Fig. 76, A). Fluid collects between the two sets of 



FIG. 76. Ovum of the rabbit at the end of the process of segmentation, oc. Outer cells, ic. Inner 

 cells, ftp. Place where the outer cells have not yet covered the inner cells. (From Balfour, after Ed. van 

 Beneden.) 



spheres, except at one part, where they remain in contact, and the ovum is con- 

 verted into a sac. formed by a layer of spheres derived from the upper primary 

 segment, and containing at one part another mass of spheres derived from the 

 lower primary segment (Fig. 76, B). The inner cells are rather more granular 

 than the outer, beneath which they gradually spread, becoming applied over a 

 part of their inner surface in a single layer : so that the cavity is afterward 

 enclosed more or less completely in a double layer of cells. 



The ultimate destination of the outermost complete layer of spheres is at 

 present doubtful. That portion of it which covers the inner cells is believed to be 

 transitory and to gradually disappear in the course of formation of the various 

 layers of the blastodermic vesicle, while the remainder forms the outer layer 

 (epiblast) of this vesicle. Adopting this view, the ovum would consist of a cavity 

 surrounded by (1) a layer of cells completely lining the interior of the vitelline 

 membrane, and (2) by a second layer internal to these and partially lining the 

 interior of the outer layer, both sets of cells derived from the segmentation of the 

 ovum. The sphere formed by this double layer of cells is called the 4k blastoder- 

 mic vesicle.' 



At first the area of the blastodermic vesicle, which consists of both the 

 inner and outer layers of cells, is a small disk, in which the first traces of the 

 embryo are seen ; hence it is called the cn'rmhial disk or area germinativa (Fig. 



