104 THE EGGS OF MAMMALS 



ture 22 hours. It was subsequently replaced, and when 

 examined 24 hours later had formed eight cells quite regular 

 in appearance. Note is made of this fact because it indicates 

 that ova segmenting irregularly at the first division may 

 eventually assume an appearance characteristic of ova under- 

 going quite regular division. The ovum of Figure 4 was 

 photographed just as segmentation from two to three cells 

 was being completed. One of the two blastomeres had not 

 quite rounded out at the time of photographing. The 

 segmented ovum of Figure 5 is also in three cells. When 

 first examined after 23 hours of culturing no segmentation 

 had occurred; 5 hours later the ovum had divided as photo- 

 graphed. The ova of Figure 5 were recovered at 18 hours 

 and 15 minutes after copulation and were still surrounded 

 by a number of follicle cells. They were placed vis-a-vis 

 in culture and the out-growing follicle cells of each ovum 

 became intermingled and caused the compression of the ova 

 seen in the photograph. Figure 6 is a photograph of a typical 

 4-celled stage, exactly comparable to the 4-celled stage 

 of fertilized ova (see Plate VII, Figs. 10 and 11). The num- 

 ber of polar bodies in such ova vary from one to three. 

 Again, the great majority of ova observed in four cells pre- 

 sented the regular appearance of the ovum of Figure 6. 

 Figure 7 represents an ovum containing seven cells in which 

 one of the four blastomeres of the 4-celled stage divided 

 twice while the others remained quiescent. Such differential 

 division may begin after the 2-celled stage as illustrated 

 by Figure 8, in which one of the original two cells has re- 

 mained quiescent while the other divided in two, and one 

 of the two cells formed divided twice to form four small 

 cells. There is also photographed the single polar body of 

 this ovum. Figure 9 represents another case in which one 

 of the early blastomeres has remained quiescent while the 

 others have gone on dividing at a rapid rate. Some such 

 process is responsible for most of the irregular segmentations 

 observed. At the same time segmentation may proceed in a 

 manner comparable to that of normal fertilized ova in vivo, 



