70 PROCEEDINGS OF THE AMERICAN ACADKMY. 



been exhausted for numbering the mice, the individuals of the second 

 hundred may be marked in like manner, care being taken to avoid giving 

 the same number to two mice of the same color. For numbers of the 

 third hundred, a round hole may be punched in the centre of the left ear, 

 and for those of the fourth hundred, a similar hole in the right ear. 

 With care, these marks may be accurately placed, and this with no 

 apparent inconvenience to the animals. The mice may be recorded 

 serially in a blank-book kept for the purpose, and the writer has also 

 found it of advantage to use a separate card for each animal. For this 

 purpose library cards may be used, the animal's pedigree entered on the 

 back, and data as to matiugs, litters, etc., on the front. The cards may 

 be used in the form of an index, and those of the individuals in any one 

 cage can thus be readily kept together or changed about as occasion 

 requires. 



In the experience of the writer, mice are found to thrive and breed 

 best where but a single pair is kept in a cage. In common with certain 

 other mammals, these mice have a strong sense of property rights, and 

 if a strange individual is introduced into a cage, he is apt to be attacked 

 at once by the inmates. If the newcomer is not despatched forthwith, 

 he is usually kept in the farthest corners of the cage, and is not suffered 

 to enter the nests until he has shown himself capable of a good defence, 

 or until the fury of his antagonists has abated. To avoid such encounters, 

 as well as the loss of valuable animals, the writer has found it sufficient 

 simply to introduce two animals, which it is desired to mate, into a cage 

 in which neither has been living. Both recognize at once that they are 

 on strange territory, and after cautiously approaching each other and ex- 

 amining the new home, they settle down peaceably without further ado. 



The period of gestation in both mice and rats is very nearly 21 days, 

 and the female is in heat again at the birth of a litter (Sobotta, '95). The 

 young should be weaned at three weeks old, for, if allowed to remain 

 longer with the mother, they become poorly nourished and puny from over- 

 crowding of the pen. Not infrequently the male will devour the young, 

 at the time they are born, if he is not removed. More rarely a female 

 mouse will devour her entire litter. 



As the young are born blind and naked, several days must elapse 

 before it is possible to determine their coat color. Albinos can be dis- 

 tinguished at birth, however, since their eyes are unpigmented, while 

 individuals which are to become colored have dark eyes at birth, the 

 pigment showing distinctly through the overlying skin. Pigmented 

 animals having pink eyes could not, of course, be distinguished from 



