EMBRYOLOGY. SEGMENTATION OF THE EGG 



We have already suggested in a preceding chapter that fertili- 

 zation of the egg, under normal conditions, occurs in the 

 oviduct, immediately after the rupture of the ovisac and the 

 discharge of the egg into the tube. The observations upon this 

 point have been carried out largely upon the rabbit as a repre- 

 sentative of mammalia and it is assumed that the phenomena oc- 

 curring in this animal are largely typical of the entire mammalian 

 group. In the observations upon fertilization in the rabbit, cop- 

 ulation has generally ensued immediately after the doe has given 

 birth to young and from 8-12 hours prior to the rupture of the 

 Graafian follicles. Under these conditions, when the ovisacs rup- 

 ture, the spermatozoa have already passed through the uterus and 

 the oviduct and have reached the ampulla of the tube, so they may 

 at once meet the egg when it is discharged and fertilization im- 

 mediately follow. The spermatozoa may even have reached the 

 ovum while yet in the ruptured ovisac, before its discharge into 

 the Fallopian tube as shown in Fig. 20. 



Fig. 20. A fully formed ovum of the Rabbit shortly before its discharge 

 from the ovary. Marshall after BischofF. 



Fig. 21. Ovum of Rabbit from the upper end of the oviduct after ex- 

 trusion of the two polar bodies. Marshall after BischoflF. 



MO, Spermatozoon. N, nucleus or germinal vesicle. NU, Nucleolus or 

 germinal spot. PB, Polar bodies. Z, zona radiata. 



This rule is probably true in our larger domestic animals, but 

 it is possible that copulation may sometimes be delayed until 

 after the rupture of the ovisac and the discharge of the ovum 

 into the tube so that, before it meets with the spermatozoa, it 



