COMPARISON WITH ALBINOS 339 



In King's Norways the average age at which these Norway 

 females, as a group, began breeding was about eight months. 

 In later generations a number of females have cast litters when 

 from four to five months of age. In captivity the menopause 

 in the Norway strain comes when the females are about twenty 

 months old, though several females have cast litters when nearly 

 two years old. 



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

 how the wild Norway form, as trapped in Philadelphia, differs 

 from the albino rats in the colony at The Institute, a comparison 

 has been made between the two forms in respect of body length, 

 body weight, brain weight, spinal cord weight and the percentage 

 of water in both the brain and the spinal cord (Donaldson and 

 Hatai, '11) as well as the weights of several of the parts and 

 viscera. (Jackson and Lowrey, '12; Hatai, '14a.) 



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

 more 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, and has a longer sexual life (Crampe, 

 '84) it is now possible to make several further statements. 



At birth the caged Norway is somewhat heavier than the 

 stock Albino (King, '15, table 1 '24) but in their relative body 

 length and the relative weights of the brain and spinal cord, as 

 well as in the percentage of water in these two divisions of the 

 central nervous system, they are approximately alike at this age. 



The marked differences between the two forms appear later, 

 during the period of rapid growth. Grouping together the 

 general differences subsequently found, we may say that the 

 wild Norway rat is absolutely heavier, relatively slightly longer, 

 has a relatively heavier brain and a heavier spinal cord, and since 

 for the same body weight as a given Albino it is younger it has, 

 when so compared, a higher percentage of water in the central 

 nervous system. 



For the same age however, the percentages of water are nearly 

 alike; the percentage in the Norway rat being a trifle higher 

 (Donaldson and Hatai, '11). The relative weights of the ovaries, 

 testes and suprarenals are also greater (C. Watson, '07; Hatai, 



