COMPARISON WITH ALBINO 191 



trapped in Vienna however, the females were in excess. There 

 are no observations on the proportions of the sexes at birth in 

 general population, but in a special study of "extracted" Nor- 

 ways made by King (MS., '15) 56 litters from females them- 

 selves taken from litters in which the two sexes were equally 

 or nearly equally represented gave 212 males and 213 females. 



f.) Opening of eyes. Miller ('11) found the eyes to open at 16 

 or 17 days and also states that the young are weaned during the 

 sixth week. 



g.) Age of sexual maturity. Miller ('11) gives one instance of 

 a female conceiving at the age of 120 days. 



Owing to the difficulty of keeping M. norvegicus happy and 

 contented in captivity, it has not yet been possible to get a trust- 

 worthy record for increase in body weight with age in the case of 

 this form. Neither our own data (Donaldson and Hatai, '11) 

 nor those of Miller ('11) show what must be the normal rate of 

 increase in body weight. 



3. Comparison of the Norway with the Albino. To determine 

 whether the wild Norway form, as trapped in Philadelphia, dif- 

 fers in any way from the albino rats in the colony at The Wistar 

 Institute, a comparison has been made between the two forms in 

 respect to body length, body weight, brain weight, spinal cord 

 weight and the percentage of water in both the brain andthe 

 spinal cord (Donaldson and Hatai, '11) as well as the weights of 

 several of the parts and viscera. (Jackson and Lowrey, '12; 

 Hatai, '14 a.) 



In addition to the familiar facts that the Norway rat is more 

 wild and difficult to handle, more successful in escaping from cages 

 and much more given to gnawing than is the Albino, that it 

 grows bigger, breeds later, has larger litters and a longer sexual 

 life (Crampe, '84) it is now possible to make several further 

 statements. 



At birth the Norway is somewhat heavier than the stock Albino 

 (King, '15, table 1) but in their relative body length and the rela- 

 tive weights of the brain and spinal cord, as well as in the per- 

 centage of water in these two divisions of the central nervous 

 system, they are approximately alike. 



