BREEDING HABITS OF ALBINO RAT 299 



place. While growth may have started in either of the two 

 preceding groups, the greatest growth occurs with the egg in 

 this stage of development. Eggs of this group have been ob- 

 served with follicles similar to those of the two preceding groups, 

 and also with follicles of nearly the maximum size. The nuclei 

 of these eggs show a diminution in the amount of chromatin 

 present and a complete vacuolization of the nucleoli, the latter 

 showing no affinity whatever for the stain. Such an egg is shown 

 in figure 9. 



The fifth group consists of the eggs with first polar spindles. 

 The follicles here are typical, showing a slight tendency to be 

 thinner in the region where the follicle is nearest to the surface 

 of the ovary. The nucleus of the egg has disappeared, and in 

 its place lies the first polar spindle (fig. 10). 



The sixth group shows no change in the size of the follicle. 

 The first polar body has been extruded and a second polar spindle 

 formed (fig. 11). 



In all the above divisions, with the exception of the sixth group, 

 wherever a distinct zona radiata can be seen, very fine proto- 

 plasmic bridges can readily be distinguished crossing froni the 

 follicle cells to the egg. The presence of these very distinct 

 filamentous processes of the follicle cells seems to have been 

 entirely overlooked by previous investigators. 



One striking thing is to be noted with regard to the above 

 divisions — never were all six found together in one ovary at a 

 given time. As was to be expected. Group I, since it included 

 all the resting eggs, was present in all ovaries. Group II, on 

 the other hand, was seen to drop out on the appearance of Group 



IV and to reappear on the disappearance of the latter. Group 



V also appeared on the disappearance of Group IV. When 

 Group V dropped out, Group VI appeared. Group III was found 

 in all ovaries. 



From the fact that perfectly normal eggs of Groups II and 

 III were found in the ovary just at, and also just subsequent 

 to ovulation, it was evident that more than one oestrus cycle 

 was necessary for the development of the egg from the resting 

 stage to the stage of the first polar body and second polar spindle. 



