116 THE EGGS OF MAMMALS 



to the critical events. He found that the zona pellucida of 

 rat and mouse ova placed in fluids of low acidity quickly 

 disappeared (at pH 3.7 or below). In a few cases the zona 

 pellucida was dissolved in Ringer's solution with a pH as 

 high as 5.4. Deciduomata of the rat have shown pH values 

 as low as 5.7, which are, however, not below the critical 

 levels of the in vitro experiments. Pincus and Enzmann 

 (unpublished data) have taken a number of measurements 

 of the pH of pseudopregnant and pregnant endometria and 



Fig. 31. Left, normal rabbit blastocysts of the 5th day of 

 pregnancy. Right, blastocysts of the 5th day of pregnancy 

 from rabbit doe ovariectomized 18 hours after mating. 

 (From the American Journal of Physiology.) 



have never observed pH values below 6.5. Nonetheless it 

 is possible that in the small decidual crypts into which the 

 ova fall the critical acidity may be attained. 



Burdick and Pincus (1935) and Pincus and Kirsch (1936) 

 have examined this critical stage of development from a 

 somewhat different angle. Corner (1928) had noted that in 

 rabbit does in which both ovaries or all the corpora lutea 

 were removed shortly after fertilization the uterine ova 

 remained in the early blastocyst stage (see Figure 31 and 

 Tables XX to XXII), whereas in control rabbits with corpora 

 lutea normal development occurred. The degenerating 

 blastocysts were associated with an oestrus type of endo- 

 metrium, and normal growth of a progestational endome- 

 trium with implantation of embryos occurred when corpus 



