386 HELEN DEAN KING. 



as much nourishment as the other members of the litter and so 

 had not grown as rapidly. The litter of 85 5 , described above, 

 shows that in some cases the very great difference in the size 

 of the various individuals in a young litter is not due to con- 

 stitutional weakness on the part of the smaller individuals, but to 

 the fact that the smaller rats were born two or three days after 

 the larger ones. 



In the course of my experiments I have found a number of lit- 

 ters, about two weeks old, in which one or more of the individuals 

 were much smaller than the rest. Two instances will serve as 

 examples of such cases. Female 4^39 gave birth to a litter on 

 October 27, 1911. When the litter was fourteen days old it 

 was examined and found to contain twelve individuals. Ten of 

 the young rats were in a similar condition of development; they 

 weighed from 13.7 gm. to 14.4 gm. each; they were well covered 

 with hair; and their eyes were beginning to open. The other 

 two members of the litter appeared normal in every way; but 

 their eyes were closed; they had comparatively little hair; and 

 they weighed only 9.1 gm. and 9.3 gm. respectively. Judging 

 from their appearance these small individuals were not more than 

 ten days old. 



A litter belonging to female 5^19 was inspected thirteen days 

 after its birth. Nine of the ten young were in approximately 

 the same stage of development, and they weighed from 16.2 gm. 

 to 17.1 gm. each. The tenth individual seemed normal and 

 well nourished, but it weighed only 10.8 gm. and appeared more 

 immature in every way than did the other members of the litter. 



The small individuals in both of these litters were earmarked 

 in order to make their identity certain, and they were allowed 

 to grow up. At the end of two months there was no perceptible 

 difference, either in size or in general behavior, between these 

 individuals and the other members of the litter. The relatively 

 small size of these rats when they were two weeks old did not 

 mean, therefore, that they were constitutionally weaker than 

 their fellows but that they were born later. 



In litters of albino rats, especially if the litter is very large, 

 it is not uncommon to find individuals that are less vigorous 

 than the other members and that are stunted in their growth. 



