46 



THE EGGS OF MAMMALS 



ovaries lacking ova (Parkes, 1926-27). This evidence, how- 

 ever, does not prove that normally oestrin-production may 

 not be under the control of the action of pituitary hormones 

 upon the ovum itself. 



In fact, aside from the presumable atresia-inhibiting in- 

 fluence, there seems to be only one other clearly evident 

 influence of pituitary hormones upon the activities of the 



»^sfr'*-T^""' ?^ 



Fig. 15. Ovum removed from a preovulatory follicle of 

 an unmated rabbit showing the vesicular nucleus. (From 

 the Journal of Experimental Medicine.) 



ovum. That is that the production of the first polar body is 

 dependent upon stimulation by pituitary hormones. 



Since this phenomenon is of some consequence to any 

 discussion of the activation of mammalian eggs the writer, 

 in collaboration with Dr. E. V. Enzmann (Pincus and Enz- 

 mann, 1935), has undertaken an examination of the mecha- 

 nism of polar body formation in the rabbit ovary. The rabbit 

 was chosen for these experiments because it ovulates only 

 after copulation and the ova are liberated regularly between 

 93^ and 103^ hours after copulation (see Heape, 1905; 

 Walton and Hammond, 1932; Pincus, 1930; Pincus and 

 Enzmann, 1932). Furthermore, the mature ova form polar 

 bodies only after copulation. According to Heape (1905) 



