240 OUTLINES OF CHORDATE DEVELOPMENT 



comparatively non- vascular, a modified band appears; this 

 is the cicatrix, where the follicle ruptures when the ovum 

 escapes from the ovary. 



Just before the egg leaves the ovary its nucleus, lying 

 flattened against the vitelline membrane, reaches the enormous 

 diameter of about 0.3 mm. It is now known as the germinal 

 vesicle, since the condensation of its small chromatin content 

 leaves the nucleus as a large clear spot (Fig. 87, D, E). The 

 last events before ovulation are the breaking down of the 

 nuclear wall and the formation of the first polar spindle. This 

 rotates into position and the primary oocyte, prepared for its 

 first maturation division, pauses to await ovulation. 



B. THE PERIOD FROM OVULATION THROUGH FERTILIZATION 



At the time a completed egg is laid, or very shortly thereafter, 

 the region of the ostium or infundibulum of the oviduct, 

 becomes very active and seems to grasp the ovarian follicle 

 containing the primary oocyte, through muscular or ciliary 

 action, or both. The follicle then becomes ruptured, appa- 

 rently by the pressure exerted by the contraction of the infun- 

 dibulum, or by its pulling away from the region of the ovary, 

 and the oocyte, contained within the infundibulum, is withdrawn 

 from the follicle, and ovulation is accomplished. In some cases 

 it may be that the follicle is ruptured before it can be grasped 

 by the infundibulum and the freed oocyte is subsequently 

 received by the fimbriated opening. The oocyte always enters 

 the infundibulum with its chief axis transverse to the long axis 

 of the oviduct, and this relation is retained during the entire 

 passage of the ovum down the oviduct. 



The spermatozoa, after their receipt by the female, make 

 their way to the extreme upper end of the oviduct, where they 

 collect, remaining alive and capable of functioning for two 

 weeks or more. Upon its entrance into the oviduct, therefore, 

 the primary oocyte becomes bathed in a fluid containing sperm 

 cells, and fertilization immediately ensues. The details regard- 

 ing the penetration of the spermatozoa are not fully described, 



