50 



WILLIAM B. KIRKHAM 



suckling young at intervals varying, with different females, from 

 one to thirteen days. One young animal was left in each case to 

 avoid too violent a reaction upon the lactating organs, and the 

 presence of a soUtary suckHng animal has been found to have no 

 influence upon the length of the gestation period. The females 

 were all killed on the thirteenth day after the birth of the suck- 

 ling young, and any embryos then present were sectioned and 

 their age determined by reference to a check series of known age 

 from non-suckling females (Kirkham, '16). The results of this 

 experiment are set forth in table 1 and should be studied in two 

 groups. The first group, cases when the full litter was suckled 

 one to six days, embraces a period before the next set of embryos 

 were ready to implant, while the second group, the remainder of 

 the table, covers an interval when the blastulae were being in- 



TABLE 1 



Data of all mice used in an experiment to determine the effect on developing embryos 

 of removing all but one of the suckling young. Unless otherwise stated, the full 

 number of young born were suckled until removed for the purpose of the experiment. 

 Stage of development of embryos is in terms of actual age of similar embryos from 

 non-suckling females where thirteenth day post-partum equals twelfth day of 

 embryonic development. All the females were killed on the thirteenth day after the 

 birth of the suckled young 



