VESICLE GROWTH AND IMPLANTATION 117 



luteum extracts were injected 

 daily after ovariectomy 

 (Allen and Corner, 1929). 

 Burdick and Pincus (1935) 

 observed that the daily in- 

 jection of oestrone begun 

 one or two days after copu- 

 lation in unoperated rabbits 

 (100-150 rat units per day) 

 and mice (5 rat units per 

 day) resulted in the degen- 

 eration of rabbit ova in the 

 early blastocyst stages and 

 of mouse ova in late morula 

 stages, i.e., at the stages 

 during which uterine entry 

 occurs. Pincus and Kirsch 

 (1936) extended these ob- 

 servations in order to fix the 

 critical time of action of the 

 hormone. Injections of oes- 

 trone were made at various 

 periods both before and after 

 ovulation, and in the case 

 of the post-0 vulatory injec- 

 tions the uteri were exam- 

 ined at the 10th to 12th 

 days to determine the extent 

 of implantation. 



Their data presented in 

 Tables XXIII and XXIV 

 indicate clearly that the 

 minimum sterilizing dosage 

 can be given on days 3 to 4 

 post coitum. These days 

 cover the period of early 

 blastocyst development. The 



TABLE XX 



