1152 



EMBRYOLOGY. 



the tube, or even after it has passed through the tube and reached the cavity of 

 the uterus ; abnormally it may take place in the peritoneal cavity. 



2. Segmentation. The first result of fertilization is a cleavage or subdivision 

 of the ovum, which is first cleft into two masses, the segmentation nucleus having 



FIG 691 First stages of segmentation of a mammalian ovum : semi-diagrammatic. (From a drawing by 

 Allen Thomson.) z.p. Zona pellucida. p. gl. Polar globules. . Epiblastic cell. 1. Hypoplastic cell. a. 

 Division into two spheres, b. Stage of four spheres, c. Eight spheres, the epiblastic cells partially enclosing 

 the hypoblastic cells, d, e. Succeeding stages of segmentation, showing the more rapid division of the epi- 

 blastic cells and the enclosure of the hypoblastic cells by them. 



previously split up into two ; so that it now consists of two separate masses of 

 protoplasm, each containing a nucleus, and situated within the original zona 

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



FIG. 692. Ovum of the rabbit at the end of the process of segmentation, oc. Primitive epiblast. ic. Primi- 

 tive hypoblast. bp. Place where the epiblast has not yet grown over the hypoblast. (From Balfour, after Ed- 

 van Beneden.) 



two divides in like manner, and thus four are formed, and so on, until at length 

 a mulberry-like agglomeration of nucleated masses of protoplasm results (Fig. 

 691). These masses are sometimes termed vitelline spheres. 



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

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

 of distinction may be called the upper, is slightly larger and paler than the other, 

 or lower. After they have subdivided three or four times the rate of cleavage in 

 the cells derived from the upper becomes more rapid than that in the cells derived 

 from the lower. In addition, the upper cells have a tendency to spread over and 



