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THE AMERICAN NATURALIST [Vol. XLV 



11 13.8 Heels^lmost black f rest "of coloring the same as yesterday; 



one seen sitting up and washing its face. 



12 14.8 Lower incisors appear (all the incisors appear on the same 



than upper). 



13 15.1 All the dorsal parts are brown. 



14 16.0 



15 16.9 



16 17.9 Eyes beginning to open; one young attempted to bite me. 



17 18.8 All have their eyes open; color the same as that of the adults 



except that it is darker and softer. 



18 19.8 



20 21.0 



21 20.9 Found the young hungry and when given a dog-biscuit, they 



incisors, and judging from their weights, yesterday was 

 the first day they did not have food they could eat. 



22 23.0 



23 23.9 



24 24.9 



out the initial loss of weight astound in human infants, 

 by Minot. g 



During the sixth week the young are weaned. At this 

 time they weigh fifty to seventy grams, and are able to 

 take care of themselves, provided food is abundant. I 

 have seen a number of rats of about this age and size 

 wandering about, in or around farm buildings, appar- 

 ently, in a starved condition. Presumably, these had 

 just been weaned and were unable to find suitable food. 



If food has been abundant the males, by the end of the 

 sixth month, weigh 230 to 290 grams while the females 

 weigh 170 to 240 grams. The male and female of one 

 of my pairs, kept throughout the year and which I 

 judged to lie in their third month when caught, weigh 

 337 grams and 223 grams, respectively. My old male 

 No. 5 weighs 460 grams and my oldest female, 345 grams. 

 These data indicate that a rat does not reach its full 

 growth before the end of the eighteenth month. 



