spherical yolk layer, this latter comes together again behind 

 them. In this condition the ovum reaches the uterus. 



III. THE SECOND DAY 



Stage 2. The first cleavage. The extrusion of yolk. 



The first cleavage. During the first half of the second day 

 after mating the fertilized ovum reaches the uterus in the 



Fig. 7 Ovum with complete albumen layer and shell membrane (X150). 

 The pronuclei have migrated to the center of the ovum. In this figure the ovum 

 proper is taken from specimen 16244; but the dimensions of the albumen and 

 shell represent the average dimensions in fresh specimens. After dehydration 

 the albumen shrinks considerably and the shell wrinkles (see fig. 10). 



pronuclear stage. The fusion of the pronuclei and the forma- 

 tion of the spindle for the first cleavage occur at the 

 center of the egg. I have studied two eggs which show this 

 cleavage in process. The first is a late anaphase (no. 16265) 



