THE EFFECT OF PROGESTERONE AND 



LACTOGENIC HORMONE UPON 



PROLONGATION OF PREGNANCY IN THE 



LACTATING MOUSE 



As EARLY as fifty years ago Lataste^ made the interesting observation that in 

 1\. some rodents (three species of gerbil, the brown rat, and the house mouse) 

 the period of gestation was prolonged if the mother simukaneously lactated 

 young from the previous litter. In studying the phenomenon more closely in 

 the mouse, he observed that the delay of parturition was correlated with the 

 number of young lactated, each young prolonging the gestation by approxi- 

 mately one day. From his experiments Lataste concluded that after the ova 

 had reached the uterus there followed during lactation an arrest in their de- 

 velopment, which equaled in length the prolongation of pregnancy. After 

 this rest period, the gestation proceeded in a normal way, and the young born 

 showed a normal stage of development. 



Curiously enough, this pioneer work of Lataste seems to have passed largely 

 unnoticed. Of recent investigators Brambell" is the only one who makes any 

 reference to it. Later workers (DanieP; King'; Kirkham" ''; Long and Evans"; 

 Mirskaia and Crew"; Enzmann, Saphir and Pincus'; Hain^"; and Brambell") 

 have generally confirmed Lataste's findings, although in certain points their 

 results differ to some extent. 



As to the mechanism of the prolongation of pregnancy it was already indi- 

 cated by Lataste's early observations that the prolongation of pregnancy dur- 

 ing lactation was due to a delay in the implantation process. This view was 

 given more support by Kirkham's experiments, and it w^as conclusively proven 

 by Enzmann, Saphir and Pincus in their careful study. By flushing the uterine 

 horns with Ringer's solution, these investigators ■were able to recover free 

 blastocysts at any time until the eighteenth day and to show that in the lactat- 

 ing females implantation occurred between the sixth and sixteenth days, thus 

 being delayed by one to eleven days. 



The hormonal factors controlling this delayed implantation have so far 

 received but little attention. TeeP' was able to produce delayed implantation 

 in nonlactating rats by injecting anterior hypophysial extract early in the 

 pregnancy. W'islocki and Goodman'" attempted to carry out similar experi- 

 ments in the rabbit, but without success. Hamlett,^^ on the other hand, tried 

 to shorten the naturally very long free blastocyst stage in the Texas armadillo 

 (Tatusia novemcincta), but his results were likewise negative. Pincus'' injected 

 estrone-free corpus luteum hormone into lactating mice and observed that 

 parturition in the treated animals occurred even later than would be expected 



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