54 



WILLIAM B. KIRKHAM 



TABLE 3 



Data regarding the length of the gestation period in mice simultaneously pregnant 

 and suckling young. The gestation period in non-suckling females averages 

 twenty days 



ment, nor, as a consequence, is the period of gestation a matter 

 that can be figured out in advance, except with broad hmitations 

 (table 3). 



SUMMARY 



1. In mice simultaneously suckling and pregnant the removal 

 of all but one of the suckling young at any time during the first 

 six days after the birth of the suckling litter leads in some in- 

 stances to implantation of the embryos as soon as they reach 

 the uterus; in other instances the implantation is more or less 

 delayed. These varying results can be correlated neither with 

 the time of removal nor with the number of young taken away. 



2. In mice simultaneously suckling and pregnant the removal 

 of all but one of the suckling young at any time from seven to 

 fourteen days after the birth of the suckling litter regularly re- 

 sults in implantation being delayed, but the exact extent of this 

 delay can be correlated neither with the exact time of removal 

 nor with the number of young removed, although removal during 

 the earlier part of the period in question does hasten implanta- 

 tion as compared with the time required if the young had con- 

 tinued to suckle. 



3. The facts in the above paragraphs justify the statement 

 that full activity of the mammary glands is the chief cause of 



