author's abstract Op this paper issued by 

 the bibliographic service, september 16 



OBSERVATIONS ON THE RELATION BETWEEN SUCK- 

 LING AND THE RATE OF EMBRYONIC 

 DEVELOPMENT IN MICE 



WILLIAM B. KIRKHAM 



Osborn Zoological Laboratory, Yale University 



INTRODUCTION 



The present paper embodies the results of two separate but 

 simultaneously conducted experiments, both designed to eluci- 

 date further the problems presented by the lengthened gesta- 

 tion period in mice which are pregnant and at the same time suck- 

 ling a previous litter. Daniel ('10), working with such animals, 

 obtained results which showed an almost constant relation of one 

 day added to the gestation period for each animal suckled; the 

 present writer ('16), however, in a somewhat similar series of 

 experiments, obtained results which showed no correlation be- 

 tween the length of the gestation period and either the number 

 of young suckled or the number of embryos carried (cf . also table 

 3). This work did, however, reveal the fact that when more 

 than two young are being suckled the implantation of embryos 

 instead of occurring on the fifth day after fertilization usually is 

 delayed until the fourteenth day, during which period the blas- 

 tulae lie free in the lumen of the uterus. The cause of this de- 

 layed implantation was tentatively stated in that paper to be due 

 to an inhibition of some sort exerted by the fully activated mam- 

 mary glands upon the uterine mucosa. The testing out of this 

 hypothesis was the purpose of the following experiment. 



INTERRUPTED SUCKLING AND THE RATE OF EMBRYONIC 

 DEVELOPMENT 



The program for this experiment was to take healthy female 

 mice which had just given birth to litters, pair them for twenty- 

 four hours with healthy males, and then remove all but one of the 



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THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 27, NO. I 



