SUCKLING — EMBRYONIC DEVELOPMENT IN MICE 51 



hibited from implanting. The importance of this grouping is 

 apparent when one considers that in the first group of cases in 

 every instance an interval of from a few hours to several days 

 intervened between the end of suckling by the full litter and any 

 readiness of the embryos to implant. Embryos in the second 

 group, on the contrary, were presumably all prepared, before the 

 removal of the suckling young, to implant themselves, and did 

 so as soon as the inhibition acting upon the uterine mucosa fell 

 below a certain minimum. 



Analysis of the data, with these facts in mind, reveals the 

 interesting fact that while some sets of embryos (J 18, 21, and 

 26) are as fully developed as control series when no young were 

 being simultaneously suckled, others show a lag in development 

 of from one to four days. No such diversity of results has been 

 found by the writer in series of embryos from non-suckling fe- 

 males, and furthermore, it is evidently not directly correlated with 

 tho number of young suckled (cf. J 26 and J 27), the number 

 of embryos (cf. J 21 and J 22), or with the combination of these 

 two numbers (cf. J 18 and J 27). There remains the explanation 

 that the irregularity is due to an individual variation either in the 

 strength of an inhibitory influence exerted by the mammary 

 glands upon the uterus or in the susceptibility of the uterus to 

 such influence. Evidently some individuals are so constituted 

 metaboHcally that the inhibition is rapidly and entirely neutral- 

 ized, so that if even a short interval elapses between cessation of 

 full mammary activity and the arrival of the eggs in the uterus, 

 implantation and further embryonic development proceed as 

 though no young had been suckled, while in other individuals an 

 even longer interval still leads to delay in embryonic growth. 

 The possible existence of such an inhibitory influence of the acti- 

 vated mammary glands upon the uterine mucosa has previously 

 been shown by the experiments of Adler ('12), who found that 

 repeated injection of extracts of mammary gland into pregnant 

 guinea-pigs and rabbits arrested the development of embryos and 

 often produced abortion. 



