52 WILLIAM B, KIRKHAM 



CONSTANT NUMBER SUCKLING AND THE DEVELOPMENT OF 



EMBRYOS 



The second method of attacking the problem of prolonged 

 gestation in suckling female mice was to determine whether 

 or not, if the number of young suckled was constant, the stage of 

 embryonic development could at any given time after fertiliza- 

 tion be theoretically determined. Four was chosen as the con- 

 stant number, since it was neither the smallest size of litter known 

 to prolong gestation nor, on the other hand, was it such a large 

 number as to prevent the use of nearly all of the available material. 



Females who had just given birth to litters of four or more 

 young had the male already present removed, together with any 

 excess number of young. This was done the morning following the 

 birth of the litter, and at varying intervals thereafter the females 

 were killed, the eggs or embryos obtained from these females were 

 sectioned, and their stage of development determined, the as- 

 sembled data being shown in table 2. 



The first notable feature in table 2 is the delay in implantation, 

 no implanted embryos being found before the twelfth day after 

 ovulation, a confirmation of work published by the writer in a 

 previous paper ('16). After the twelfth day of gestation there 

 appears the same lack of correlation, as noted above in con- 

 nection with table 1, between their stage of development and 

 the time of ovulation, and since if degeneration is going to 

 occur it always takes place at the stage of four or five days' 

 development, this possible factor can be ruled out. In other 

 words, we have here proof of the fact that the irregularities in 

 the rate of development of embryos in females which are simul- 

 taneously pregnant and suckling is due, only in small measure, 

 if at all, to the number of young suckled, for if the number suckled 

 and the rate of embryonic development were correlated the 

 data in table 2 should indicate a regular correlation between 

 stage of development and age of embryos, and such is not the 

 case. 



The explanation of this failure to correlate the ratio of embry- 

 onic development in suckling females with either the size of the 



