REPRODUCTION 153 



In relation to the body surface H/W (W^ = relative body 

 surface) the heat production is high in group 1^ — young active 

 animals with a relatively large body surface; lowest in group 2 — 

 90-130 grams in body weight; and somewhat higher in group 3 — 

 see table 112 which gives: 



(I) The weight of the rat after 24 hours fast. 

 (II) The temperature of the calorimeter during the experi- 

 ment. 



(III) The time of year. 



(IV) The heat production for relative areas calculated per 



body weight (grams) ' i.e. H/W^ 

 (V) The heat production calculated per body weight 

 (grams) H/W. 



In animals living together the heat production per gram is less 

 than in the same animals living separately, and it has been ob- 

 served that when living together the rats grow faster (Hopkins, 

 '13) than when living alone. 



For the direct determinations of the area of the skin (Hill and 

 Hill, '13) see page 121." 



10. Reproduction and the sex organs. The relations between fer- 

 tility and nutrition are very evident in the rat and the existence 

 of a dietary factor essential for reproduction has been postulated, 

 Evans and Bishop ('22 d). 



Przibram has found that rats put in the cold room early in the 

 autumn and then removed to a warm room breed from then 

 throughout the winter, thus avoiding the interruption to breed- 

 ing from November to January, which occurs under the usual 

 conditions. Constancy of (moderate) temperature is in general 

 favorable to continuous reproduction. 



In the immature female the vagina is closed by a membrane. 

 This usually breaks down just before the first oestrus. Table 

 113 gives the data on these relations and also the length of the 

 first four oestrous cycles (Long and Evans '22). 



The later oestrous cycles are about five days in length (4.6-4."8). 

 In each cycle four stages are recognized (Long and Evans, '22) 



