The Genetic Factors in the Development of the Housemouse, etc. gg 



of them and to note this mark on the label. For this work it is 

 important to use cither metal or metalborded labels, for fear of damage 

 by wild or escaped mice. 



I often put seven or more females of different colours in one cage 

 with one male, sometimes as many as fifteen. The cages containing 

 a male with females were marked by a special label, and such cages 

 were inspected twice a week, to see whether any females were pregnant. 



As soon as discovered, a pregnant female was separated, and on 

 her label was hung a second one, giving the number of the male. 



My first cages were made of wood, but I soon discarded these 

 for metal ones, consisting of two stamped tin pie-pans with a five 

 inch wide strip of galvanized wirenetting between. This wirenetting 

 was attached into a circle by means of paperclips, and fitted into 

 the bottom of the lower pan, containing sawdust, the second pan 

 being used as a cover. Such cages have the advantage of being 

 easily taken apart to be thoroughly cleaned, they can be easily made 

 by anyone, from material which can always be procured, in a few- 

 moments time, and, above all, they cost only an infinitesimal sum. 

 The mice kept in them can not have their noses slashed by wild 

 mice, and they cannot gnaw the cage to pieces. I have kept very 

 many wild mice in them, without ever losing one. In cleaning the 

 animals, I transferred all the mice and labels of one cage to a clean 

 one; when extra wild ones were amongst them, I used a set of two 

 covers, united by a tube of wirenetting, putting these covers in place 

 of the ordinary ones, and making the mice pass through the tube. 

 In these metal cages the mice should have a tin receptacle in which 

 to make their nest. I have always personally cleaned and fed the 

 animals, which, I think, is the only way to avoid mistakes. 



In the catalogue the mice followed in order of their number, 

 and iu this catalogue were given the number, sex and colour of the 

 animal, the numbers of its parents, those of the animals with which 

 it was mated, and, together with these, the numbers of the 

 resulting young. 



Thus, the descendance ijf any given mouse could be traced as 

 easily as its ancestry. 



Throughout the experiments, I have, to find out the genetic 

 constitution of a given animal, always recurred to test-matings. To 

 know whether an animal, showing the presence of a factor x were 

 homozygous or heterozygous for this factor, I always mated it to an 

 XX individual. This way of finding out the genetic constitution is 



7* 



