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9 LEAFLET 253, U. S. DEPARTMENT OF AGRICULTURE 
widely distributed throughout the globe, no doubt largely because it is readily 
transported by man. Most laboratory rats are probably mutants and domes- 
ticated strains of this species. M. rattus and M. norvegicus are mutually in- 
fertile, but available albino strains breed freely with the latter species. Com- 
parisons of hemoglobin crystals and skull shape confirm their relationship. 
The albino rat has undergone modifications and the various strains which 
have been more or less inbred in different laboratories over a period of years 
show conspicuous differences in size and other characteristics important in 
animals for laboratory use. The albinos now composing the colonies in vari- 
ous laboratories are not strictly homozygous, even from the standpoint of 
color, since crosses with pigmented forms give offspring with color markings 
which could have been derived only from the albino parent. 
USE OF MICE AND RATS 
Because of its fertility, prolificacy, convenient size, short gestation period, 
variations, inexpensive maintenance, resistance or susceptibility to certain 
diseases, and ease of production, the mouse has become a favorite research 
animal. Its variability has made it a valuable animal for genetic research, and 
it has been more intensively studied in this connection than any other mam- 
mal. It is widely used in medical research, especially in work on cancer. It 
serves as a medium in which the pathologist cultures disease germs and the 
physiologist studies yarious life processes, including the interaction of the 
hormones involved in reproduction. The embryologist and anatomist have 
learned much about the early developmental processes of mammals in general 
from the mouse. Thus the mouse has contributed in wide measure to the 
biological sciences. 
The rat has shared with the mouse its reputation as an animal for re- 
search. It has been used more extensively in the field of nutritional investi- 
gations than the mouse. It maintains an important place in the study of 
hormones and in the testing and standardizing of drugs. It also has been 
a favorite animal for use in animal-psychology experimentation because of 
its relatively high learning ability as compared with other small laboratory 
animals. 
THE LABORATORY MOUSE 
Reproduction and Growth 
Ovulation occurs in the mouse at fairly regular intervals of 3 to 4 days, and 
is not dependent on copulation as it is in the rabbit and some other animals. 
The female’s first estrus occurs at 30 to 40 days of age. The sexes should be 
separated prior to this. Estrus reeccurs 6 to 24 hours after a female gives 
birth to a litter and continues for 12 hours or less. Gestation normally lasts 
19 to 21 days except in females which are nursing a litter, in which case it 
may be lengthened from 6 to 16 days. A common practice in the manage- 
ment of a mouse colony is to place three to six females in a cage with a male. 
After 15 to 18 days the females should be eXamined for signs of pregnancy, 
manifested by greatly distended abdomens. ‘Those females «which are preg- 
nant should be removed to separate cages, each female in an individual cage. 
Females usually make a nest in which the young are placed and Kept until 
they are able to run about. The average number of young is 6 to 8, but 
litters of 12 to 15 are frequent. A mouse at birth weighs about 1 to 1.5 grams, 
depending on the number in a litter, the young in smaller litters having the 
greater individual weight. At 4 months of age a mouse weighs about 25 
grams, slightly less than an ounce. 
At birth the mouse has no hair except the vibrissae, or hairs about the nos- 
trils; no pigment in the skin except in the iris of the eye, visible through closed 
eyelids; and the ears are thick, bent forward, and attached to the face and 
cheek. After 2 to 3 days hair appears, at 4 to 6 days the ears detach from the 
face, and at 8 to 10 days the mouse is fully haired. The incisor teeth erupt 
at 11 to 13 days and the eyes open at 14 to 16 days. At this time the young 
