4 LEAFLET 253, U. S. DEPARTMENT OF AGRICULTURE 
simply transferred by hand from the one to be cleaned to a clean cage. It is 
well to have enough cages or pans so that one set may be sterilized with steam 
or by dipping in a solution of disinfectant while the others are occupied. 
Both types of cages may be arranged on racks five to six tiers high. 
FEEDING 
A convenient food for mice is the compressed dog biscuit, which is well bal- 
anced as it contains minerals and essential food elements required by mice. 
A satisfactory mixture also is one composed of 240 parts of rolled oats, 30 of 
powdered skim milk, 8 of cod liver oil, and 1 of salt. 
Various other grains such as ground barley or yellow corn meal may be used 
in addition to the ones named in the mixture given above. Such foods are 
fed in a can. Various arrangements may be made for preventing the mice 
from wasting and contaminating the food. 
A piece of carrot, potato, or green leaf of cabbage or kale may be fed oc- 
casionally. These should be clean and free from decay. 
THE ALBINO LABORATORY RAT 
Reproduction and Growth 
The rat will reproduce at all seasons, but larger numbers and more vigorous 
animals are generally obtained in the spring months. Reproduction is lowest 
through the late fall and winter. The estrous cycle of the rat is 4 to 5 days, 
and the gestation period approximately 21 days. 
The young are born with closed eyes and ears and without hair. Within 
21 days they may be weaned and will develop normally when fed a suitable 
diet such as one of those given in the section on feeding. Although they vary 
some in the time required to reach sexual maturity, the average animal is 
mature at 70 to 80 days. It is generally considered good practice to delay the 
mating of these animals until they are about 4 months of age. 
One satisfactory method of mating the animals is to place several females, 
up to five or six, in a cage with one male, and remove the females to individual 
cages when they show signs of pregnancy, such as a plug, rapid increase in 
weight, or sign of erythrocyte, or blood clot. The plug consists of a mass of 
seminal fluid which has coagulated in the vagina of the female after copula- 
tion. It is expelled after a few hours and may be seen in the litter on the 
floor of the cage. The plug is not always a sure sign of pregnancy, as the 
fetuses may have died before term, but rapid increase in weight, especially 
near the end of the gestation period, is usually a good sign of pregnancy. 
However, if the number of fetuses is small, the increase in weight will be 
scarcely noticeable. The most certain sign of pregnancy, and that which is 
routinely used for determining pregnancy in the rat, is the presence of an 
erythrocyte, or blood clot, in the vagina on the thirteenth day of pregnancy. 
This may be large enough so that it protrudes and is visible from the outside; 
or the edges of the vulva may be spread slightly with forceps in order that it 
may be observed. After the end of the lactation period females should be 
rested several days, up to about 2 weeks, before remating. 
With this method females generally produce four to six litters and then 
usually are discarded. They are so near to menopause (about 15 months of 
age) that reproduction is uncertain. 
Considerable variation occurs in the number of young to the litter. From 
1 to about 10 is the usual number, the average being between 6 and 8. Ifthe 
large litters are reduced to about 7 at birth the young develop better and the 
strain of lactation on the female is less. 
Other conditions being satisfactory, this procedure should yield about half 
as Many young per week as there are female breeders in the colony. 
Sexing of young rats at weaning age and even earlier is often necessary. 
The most dependable external character by which sex may be determined at 
birth is the distance from anal opening to genital papilla. This distance is 
