SEGMENTATION OF THE EGG. 



353 



The larger cells now become grouped together in the centre, 

 while the smaller cells form a cap lying on these, and partially in- 

 closing them. In the later stages, the smaller outer cells divide 

 rather more rapidly than the larger cells, and inclose these more 

 completely ; and at the close of segmentation, about the seventieth 

 hour, when the ovum passes from the oviduct into the uterus, it 

 consists of a central solid mass of rather larger and more granular 



Fig. 136. 



Fig. 137. 



Fig. 136.— A Rabbit's Ovum from the middle of the length of t lie. oviduct, 

 about twenty-two hours after copulation, showing division of the ovum 

 into two cells. (After Bischoff.) x 200. 



CB, blastomere or segmentation cell. MO, spermatozoon Imbedded in the zona 

 radiata. W, nucleus. Z, zona radiata. 



Fig. 137. — A Babbit's Ovum from the lower end of the oviduct, about the 

 middle of the third day ; showing the morula stage, shortly before the 

 completion of segmentation. (After Bischoff.) x 200. 



cells (Fig. 138, CD), almost completely surrounded by a layer of 

 rather smaller and more transparent cells, slightly flattened at 

 their outer ends (Fig. 138, cc) ; the larger cells being visible on 

 the surface at one spot only. 



In the size of the eggs there is a close agreement between 

 the rabbit and Amphioxus ; the rabbit's ovum measuring on an 

 average 01 16 mm. in diameter, and that of Amphioxus measuring 

 0*104 mm. The two eggs agree also in undergoing complete 

 or holoblastic segmentation, and in the blastomeres, or cells 

 formed by segmentation, differing very little from one another 

 in size. 



The comparison, however, must not be pushed too far. The 

 actual arrangement of the cells is entirely different in the two 

 cases : the rabbit's ovum does not pass through a gastrula stage 

 (cf. Figs. 15 and 16) ; and there is no stage in the development 

 of Amphioxus similar to that represented for the rabbit in Fig. 

 138. 



A A 



y 



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