70 THE EGGS OF MAMMALS 



As they separate out of the cuinuhis mass the egjj;s emerge 

 surrounded more or less by a few adherent folUele cells, and 

 proceed down the tubes where these few adherent cells are 

 lost. At 20 hours after copulation all the adherent cells are 

 gone and a thin layer of albumen is laid down about the 

 zona pellucida. Eggs washed out at this time show very 

 clearly the transparent, shining zona pellucida about the 

 yolky, granular egg cytoplasm, with an extremely thin al- 

 bumen layer surrounding the zona (see Figure 4). The 

 process in\'oh'ing the separation of the eggs out of the 

 cumulus mass and the clearing off of adherent cells 

 thus involves a period of about 3 hours. When eggs are 

 washed out during this period one observes in a single 

 washing all the stages described, eggs completely clear 

 of adherent cells being preponderant toward the end of 

 the period. One may even find an occasional egg still 

 surrounded by adherent cells as late as 20 hours after cop- 

 ulation. 



It is important for reasons that will be obvious later, 

 to note that by 20 hours after copulation all rabbit ova are 

 free of follicle cells and have begun to accumulate a layer 

 of albumen. By 24 hours after copulation this albumen 

 lajTr is quite appreciable (see Figure 5). Subsequently the 

 ova descend to the uterine end of the tubes acquiring in their 

 passage successive layers of albumen so that the albumen 

 layer may eventually become several times the thickness of 

 the egg itself (see Figures 6 to 9). The zona pellucida no 

 longer presents the clear, shining appearance observed before 

 the deposition of albumen. Most of the ova recovered from 

 the tubes contain at least one polar body, occasionally two 

 or even three. In some cases none ha\'e been observed but 

 this may be ascribed to faulty observation as the eggs often 

 come to rest with the polar body hidden. 



The eggs enter the uterus between 72 and 96 hours after 

 copulation. No more albumen is added and the eggs undergo 

 rapid disintegration. It is, in fact, very difficult to recover 

 unfertilized ova from the uterus. Pincus (1930) was unable 



