Segmentation of the Egg 



283 



authors estimate that in woman the period is probably five to 

 eight days, but we are aware of no definite data upon which to 

 base this assumption. 



About 10-12 hours after the fertilization of the egg of the 

 rabbit, the ovum undergoes cleavage, by which there arise two 

 spherical cells, which are essentially alike in all respects except 

 that some observers believe that the one is slightly smaller than 

 the other. See Fig. 22. 



Each of these two cells, after a brief pause of a few hours, 

 divides again into two cells, constituting a mass of 4 ovoid cells, 

 which again subdivide to constitute a group of 8 cells, those de- 

 rived from the larger of the two first cells being now more clearly 

 larger than the others, the larger ones grouped together cen- 

 trally, while the smaller rest upon them as a cap. Later the 

 segmentation of the small cells proceeds somewhat more rapidly 

 than that of the larger and they tend to grow around and enclose 

 the latter. According to Marshall, when the ovum of the rabbit 



Fig. 23. A rabit'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. X200. Marshall after Bishoff . 

 Fig. 24. A rabbit's ovum seventy hours after copulation, taken from the 

 lower end of the oviduct just before entering the uterus and showing the 

 condition at the close of segmentation. X 200. Marshall after Van- 

 Beneden. 

 has reached the 70th hour after fertilization, its segmentation 

 has been completed and it passes from the oviduct into the uterus. 

 At this time it is a spherical mass consisting of an exterior layer 

 of small, nearly spherical, transparent cells, enclosing almost 

 completely the group of larger, more granular cells ; it has 

 reached what is known as the morula or mulberry stage. See 

 Figs. 23 and 24. 



