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THE AMERICAN NATURALIST [Vol.LVI 



found that after some practise heat could be determined 

 rather accurately by inspection. A guinea pig in rut 

 will often assume the position of copulation when stroked 

 gently over the lumbar region. The vulva are swollen 

 and moist, and often a cheesy secretion is seen. The 

 latter is a positive sign of heat, but we found that some 

 guinea pigs refused to mate during the entire period in 

 which the secretion was present. 



In young animals we found heat recurring every fifteen 

 or sixteen days with very little variation among indi- 

 viduals of the same age. Three striking exceptions in 

 which heat returned in twelve days will be reported later 

 in this paper. Papanicolaou and Stockard found that in 

 old multiparge the period may be lengthened to 18 days. 

 I also found that as the animals grow older they seem to 

 become more and more irregular in their rhythm. In 

 three very old animals I was unable to find any signs of 

 heat throughout an entire year, although I attempted to 

 mate them twice daily. Three other animals maintained 

 a cycle of 20 days, and in some cases we were unable to 

 demonstrate any regular cestrous rhythm at all, either by 

 inspection or by the use of a male. 



Subsequently (1920) these workers have reported that 



underfeeding with a diet of 20 grams of carrots per 

 day produces prolongation of the dicestrum, and at the 

 same time a congestion in the ovary and uterus and a de- 

 generation of developing Graafian follicles." They con- 

 cluded that the extent of prolongation of the dicestrum 

 depends upon the stage at which an animal is underfed. 

 . . . Large follicles seem to require better nutrition than 

 a small primary follicle. . . . Thus a late underfeeding 

 has a more injurious effect than an early one, and post- 

 ponement of the next oestrus is correlated with a postpone- 

 ment of new ripe follicles in the ovary." Stockard and 

 Papanicolaou believe that the ovarian follicles are ex- 

 tremely sensitive to environmental conditions. They be- 

 lieve that extreme variations in the oestrus cycle of cer- 

 tain animals may be accounted for, partially at least, by 

 differences in nutrition. 



In the course of thes^ observations the intervals be- 



