94 THE EGGS OF MAMMALS 



tion from the oestral to the luteal phase. Furthermore, it 

 is possible that the ovarian hormones themselves may di- 

 rectly affect the cleavage process. Oestrin, for example, 

 definitely stimulates the mitotic activity of the vaginal 

 epithelium, progestin inhibits uterine mitoses, etc. 



Accordingly Burdick and Pincus (1935; also Pincus and 

 Kirsch, 1936) have investigated the effect of ovarian hor- 

 mones upon the development of rabbit and mouse ova. 

 They found that the injection of large amounts of oestrin 

 in no way affected the cleavage process although ova in the 

 early uterine stages degenerate and die when only moderate 

 amounts of this hormone are injected (see Tables XXIII 

 to XXV, pages 118-120, 122). That the hormone injected 

 definitely affected the tubal tissue was evidenced by the fact 

 that in both mice and rabbits an effective closure of the 

 tubo-uterine junction was attained, and in rabbits both 

 the contractile activity and the histological appearance of 

 the tubal tissue were definitely altered to the oestrus type. 

 In addition (Pincus and Kirsch, 1936) it was found that 

 rabbit ova grow^n in cultures containing appreciable amounts 

 of oestrin continued to cleave at the normal rate. Finally 

 fertilized rabbit ova in 1- and 2-cell stages were injected 

 into the fallopian tubes of does on heat (and therefore 

 lacking corpora lutea), and these were found to develop 

 normally up to the early blastocyst stage. Corner (1928) 

 had already shown that in bilaterally ovariectomized rabbit 

 does egg development stops at the early blastocyst stage. 

 The segmentation processes appear, therefore, to be inde- 

 pendent of the secretory activity of the ovaries, and of any 

 effect that the ovarian condition may have upon tubal 

 secretion. Rabbit ova will, indeed, go through the morula 

 stage in a carefully balanced buffered Ringer-Locke solu- 

 tion, indicating a fairly complete lack of dependence upon 

 any special organic nutrition. It has, of course, been re- 

 peatedly noted by observers of living material {e.g., van Ben- 

 eden, 1875; Gregory, 1930; Gilchrist and Pincus, 1932; 

 Squier, 1932) and by those who have examined fixed speci- 



