VESICLE GROWTH AND IMPLANTATION 123 



Just what special conditions are needed for carrying the 

 blastodermic vesicle over this critical stage cannot be ex- 

 plicitly stated. It is obvious that corpus luteum activity 

 is necessary for the establishment of these conditions, and 

 the oestrone effect is due to an inhibition of this activity. 

 Thus it is possible to overcome the partially sterilizing ef- 

 fect of low oestrone dosages by the simultaneous injection 

 of a corpus luteum hormone preparation (Pincus and Kirsch, 

 unpublished data). Other substances {e.g., vitamins A and 

 C) are ineffective as inhibitors of complete sterilization. 

 There seem to be tw^o alternatives: either (1) progesterone 

 or some corpus luteum product act directly upon the blasto- 

 cysts or (2) corpus luteum secretions induce a special uterine 

 environment through their action upon the endometrium. 

 Pincus and Enzmann (unpubhshed data) have made crude 

 extracts of the endometrium of pseudopregnant rabbit does, 

 and have cultured blastocysts in media containing these 

 extracts. No marked effect was obtained with the particular 

 preparations employed, but further investigation may dis- 

 close the presence of an active substance. It is certain that 

 blastocyst death due to oestrone action occurs in a uterus 

 the endometrium of which still shows at least partial pseudo^ 

 pregnant proliferation. The minimum sterilizing dosage 

 employed by Pincus and Kirsch is insufficient to abolish 

 pseudopregnant growth completely (Leonard, Hisaw and 

 Fevold, 1931; Courrier and Raynaud, 1933). Courrier and 

 Raynaud (1934) have also found that dosages sufficient to 

 prevent implantation are below the level necessary for the 

 abolition of pseudopregnant growth. The data presented 

 here on sub-sterilizing dosages demonstrate explicitly that a 

 certain number of vesicles fail to develop in a uterus in 

 which others proceed normally. We may consider therefore 

 that there is necessary at least a threshold amount of a 

 necessary active substance, or an optimum-hydrogen ion 

 concentration alterations of which differentially affect the 

 various blastocysts, or a rate of uterine contraction which 

 causes the proper lodging of the blastocysts in the endo- 



