THE EARLY EMBRYOLOGY OF THE MOUSE 



out the vitellus, and at the hrst cleavage division are divided more or less 

 equally, alono; with the mitochondria already present in the egg, to the two 



Follicular cells 



Vitellus 



Zona pellucida 



Zona pellucida 



Polar body 



Perivitelline space 



B 



Fig. I. — rhotugraphs of mouse eggs (X6oo). A. Egg removed from ovary. 

 B. Fertilized egg from oviduct 20 hours after copulation. Two polar bodies and 

 sperm in perivitelline space. {From Lewis and Wright.) 



daughter cells. There is some evidence that Golgi material is also carried by 

 the sperm into the egg (19, 22, 34). 



The sperm and egg nuclei, now both within the vitellus, are known as the 

 male and female pronuclei. They move towards each other until they lie 



