Cyclic Changes in Mammary Gland of Guinea Pig. 165 



that the experiments of Starling and others concerning the source 

 of the growth substance which acts on the mammary gland are not 

 conclusive. 



Our investigations concern the cyclic changes in the mammary 

 gland of the guinea pig. We studied the mammary glands as well 

 as uterus and ovaries in 262 animals, in almost all of which the 

 time of ovulation had been ascertained prior to the experiment. 

 In many of these animals the effect of ovaries and uterus on the 

 cyclic changes was analyzed by various experimental procedures. 



Without going into a detailed discussion of our results, we 

 may state our principal conclusion as follows: The normal sexual 

 cycle of the guinea pig (the period between two ovulations), has 

 a duration of approximately 16-18 days. We can also, in the case 

 of the mammary gland, distinguish two phases in this cycle — one 

 comprising the time of heat and ovulation and two or three days 

 following ovulation; in the large majority of cases the mammary 

 gland proliferates mitotically during this phase. In the second 

 phase, comprising the remainder of the sexual cycle, proliferation 

 is as a rule absent. Only toward the end of this phase, from the 

 fifteenth to the twentieth day, we find again in some cases pro- 

 liferation. The first proliferating phase depends upon the ab- 

 sence of the corpus luteum. We can accelerate it by an early 

 extirpation of the corpora lutea, in a way similar to the acceleration 

 of ovulation and uterine heat changes by the same procedure. 

 The corpus luteum of the ordinary sexual period in the guinea pig 

 does not usually produce proliferation of the mammary gland. 

 Also during pregnancy, which lasts in the guinea pig about twice 

 as long as in the rabbit, proliferation of the mammary gland 

 occurs regularly only after the twenty-fourth day of pregnancy. 



If through certain experimental procedures we prolong the 

 sexual cycle, we find usually a proliferating gland in cases in which 

 well-developed living deciduomata and good corpora lutea, or 

 in which strongly developed, not degenerated corpora lutea with- 

 out deciduomata, are present. In those cases in which during the 

 period of prolongation deciduomata and corpora lutea are degen- 

 erating, proliferation of the mammary gland as a rule is absent. 



In case of castration and of the presence of hypotypical 

 ovaries, proliferation of the mammary gland is not found. Con- 



