20 



HUMAN ANATOMY. 



the entrance of the favored spermatozoon into the substance of the ovum, an effec- 

 tual barrier to the penetration of additional seminal cells is presented by the thick 

 vitelline membrane which immediately forms. The point at which the spermatozoon 

 is about to enter the egg is indicated by a conical elevation, the receptive eminence, 

 into which the male germ-cell sinks, — the tail only partly entering the protoplasm 

 of the egg and very soon disappearing. 



The position of the remains of the spermatozoon within the substance of the 

 ovum is indicated by an ovoid body, the male pronucleiis, which contains the chro- 

 matin and centrosome of the paternal germ-cell. The sperm-yiucleus and the egg- 

 nucleus, as the male and female pronuclei are now often designated, usually break 

 up into their respective chromosomes without fusing into a single segmentation 



Fig. i6. 

 B C 



O 



£i 



Early stages of segmentation as seen in ova of mouse, surface view. X 450- {Sobotta.) The external double 

 contour represents the zona pellucida; the cell marked with X, the polar body. A, fertilized ovum at stage of the 

 pronuclei ; £, two segmentation spheres of equal size ; C, segmentation spheres of unequal size ; D, three-cell stage 

 resulting from division of larger sphere ; £:, stage of four spheres; /=", six; C, eight; //", sixteen; /.twenty-five. 



nucleus. In this case the two groups of chromosomes unite in the first mitotic 

 figure, the segmentation spindle (Fig. 17). 



After the fusion of the pronuclei, and just as segmentation is beginning, the 

 fertilized ovum presents a clear oval area which contains the two groups of chromo- 

 somes contributed by the germ-ceUs of both parents ; on opposite sides of the chro- 

 matin figure are the centrospheres, each containing a centrosome and surrounded 

 by a marked polar striation within the substance of the egg. The centrosomes now 

 present within the ovum are usually both derived from the substance of the cen- 

 trosome of the spermatid, which entered the ovum as the neck-granules within the 

 middle-piece of the fertilizing spermatozoon. The r61e of the latter, therefore, is two- 

 fold, — to contribute the chromatin necessary to restore to the parent cell the normal 



