274 J. J. Christian 



was based on the fact that splenic hypertrophy, which had been observed 

 previously only in natural populations of high density, had made its ap- 

 pearance recently in laboratory stocks of these voles. The only factor which 

 seemed to account for this appearance was the introduction into the labora- 

 tory breeding population of voles from natural populations exhibiting 

 splenic enlargement. The fact that a similar hypertrophj' was observed in 

 response to increased population density in a highly inbred strain of mice, 

 maintained under constant conditions for a good many years with no expo- 

 sure to natiu-al conditions, makes a genetic explanation of the sort postu- 

 lated by D. Chitty unlikely in such an inbred strain of presumptively gene- 

 tically stable mice. Similarly, as was pointed out earlier, a comparable 

 hypertrophy of the spleen was observed in a few inbred rats subjected to 

 alarming stimuli. The hypertrophy, when examined critically, has been 

 found to be due to increased hematopoiesis (Dawson, 1956), possibly in 

 response to the increased stimulation of erythropoiesis by hormonal factors. 

 It is also well known that splenic erythropoiesis can be stimulated by any 

 stimulus that produces anemia. Therefore, conclusions regarding the causes 

 of splenic hypertrophy, due to increased erythropoiesis, must be inter- 

 preted with caution; although it does not seem likely that it is genetic in 

 origin, at least in inbred albino mice with splenic hypertrophy following 

 increased population density. 



4. Reproductive Fuxctiox in Female Mice: Lactation, Reproduc- 

 tion 



Female mammals have frequently been observed to respond to adverse 

 stimuli with a reduction of reproductive function. When rats are suddenly 

 moved from a temperature to which they have become accustomed to a 

 different temperature, either higher or lower, there is retardation of growth 

 and prolongation of the estrous cycle (Bohanan, 1939). Selye (1939) has 

 listed a variety of agents which will inhibit o^'arian function and estrus. 

 Reproduction is also depressed in female mice by increased population 

 density. We have noted that Crew and Mirskaia (1931) and Retzlaff (1938) 

 found that reproductive performance of female albino mice declined with 

 increasing population density. In another series of experiments no young 

 were produced and no females became visibly pregnant when mice were 

 crowded 20 males and 20 females to a cage for 6 weeks (Christian and Le- 

 Munyan, 1958) . It is not known whether there was a marked suppression 

 of ovarian function with diminished ovulation, a failure of the ova to im- 

 plant, or intra-uterine loss early in pregnancy, but more than likely all 

 these factors were involved. When the population size was reduced to 10 

 males and 10 females, all the females became pregnant, but the number of 



