VESICLE GROWTH AND IMPLANTATION 127 



tion occurs in the luteal cell cytoplasm, and the removal 

 of such corpora lutea leads to abortion whereas removal 

 during the free vesicle period has no discernible effect upon 

 the uterus or ovum. Hamlett (1935) quotes a number of 

 instances of naturally-occurring delayed implantation of a 

 presumably similar nature. 



This possibility has been tested by injecting oest rone-free 

 corpus luteum extracts into lactating pregnant mice during 

 the early part of pregnancy (unpublished data). Injections 

 of approximately l/20th of a Corner- Allen rabbit unit were 

 made over a 5 to 8 day period. A number of the mice failed 

 to produce any young but seven females gave birth to normal 

 litters. These were born not at term but much later; in 

 fact, the average date of birth was 4 days later than would 

 occur in delayed pregnancy if the expected delay is calculated 

 on the basis of 21 hours per suckling young. 



The implication is clear that excessive corpus luteum 

 secretion caused a delay of pregnancy in mice. Since Teel 

 (1926) found that deciduomata formation could be readily 

 induced in the uteri of unmated females treated with his 

 extracts corpus luteum activity undoubtedly occurred as a 

 result of luteinizing hormone injection. The act of suckling 

 then, by prolonging corpus luteum activity (which it does — 

 see Parkes, 1929; Turner, 1932), results in a delay of im- 

 plantation. Selye and McKeown (1934a) have in fact shown 

 that suckling in rats prolongs pseudopregnancy and that 

 the effects of suckling do not occur in the absence of the 

 ovary (Selye and McKeown, 19346). 



The fact that Teel obtained definite deciduomata in ani- 

 mals subjected to a treatment that produces delayed preg- 

 nancy indicates either: (1) that mechanical irritation is 

 more effective than ovum contact and that therefore the 

 corpus luteum effect is really subnormal or (2) that excessive 

 corpus luteum activity in some way inhibits the actual 

 process of implantation of the blastocysts. The problem is 

 an interesting one and is receiving further investigation. 



