678 



THE OVUM AXD BLASTODEKM. 



just in proportion to the respective limits of the germinal and nutritive 

 parts of the yolk ; but always affecting first the germinal part, and 

 extending subsequently outwards from the germinal pole as a centre. 



The process of segmentation has not been seen in the human subject, 

 for the human ovum has not yet been detected in the progress of its 

 descent through the Fallopian tubes ; but the phenomena have been 

 observed with care by Bischoff and others in a variety of marami- 

 ferous animals, and as no important differences have been found to 

 occur among them, there is no reason to doubt the similarity of the 

 process in man. The yolk cleavage sets in within a few hours of the 

 entrance of the mammiferous ovum into the tube, and continues to pro- 

 gress regularly during its descent towards the uterus, soon after its 

 arrival in which the process is completed. The duration of this varies 

 in different animals, being not more than from three to four days in the 

 rabbit, in which it is the shortest known, and extending to from seven 

 to eight days in the dog. It probably occupies not less than eight days 

 in the human subject. 



In the bird's egg the segmentation of the cicatricula is accomplished 

 between the time of the entrance of the yolk into the oviduct and that 

 of its being laid with its albuminous, membranous, and shell coverings, 

 which may vary from IG or 20 to 24 or 30 hours ; and there may be 

 some difference in the degree of completeness of the segmenting pro- 

 cess at the time of the exclusion of the egg, according to the time the 

 egg has taken to pass through the oviduct, the season of the year, and 

 other circumstances. 



Fig. 493. Fig. 493.— OvrM of the Rabbit 



SIXTY-EIGHT HOURS AFTER IM- 

 PREGNATION (Allen Thomson). 

 2 so 

 1 ■ 



This ovimi is probably in tlie 

 sixth stage of segmentation. Sper- 

 matozoa were observed within the 

 zona, z, the zona ; a, the thick 

 layer of albumen peculiar to the 

 rabbit's ovum at this stage. 



a. Segmentation of the 

 mammaVs ovum. — This pro- 

 cess may be shortly de- 

 scribed as follows : — First 

 the whole mass of yolk- 

 protoplasm, contracted as 

 before mentioned, splits into 

 two somewhat ovoid or el- 

 lipsoid masses, by the for- 

 mation of a fissure which 

 begins on the surface and 

 speedily runs through the 

 whole thickness of the yolk 

 (fig. 492, b). The two masses so formed lie somewhat pressed together 

 within the vitelline membrane ; each mass presenting nearly the same 

 appearance and structure as the whole yolk did previous to its cleavage. 

 But as soon as this change has taken place, and according to some 



