100 Hagedoorn. 



vastly superior over a haphazard mating inter se of hybrids from a 

 cross. It is, morover, a great saving of time and material, for, to 

 recognize an Xx by mating it to another Xx, one needs twice as 

 manj' young as from the testmating Xx with xx, the first mating 

 giving 25 °o' the second one 50% recessives. In the course of the 

 experiments I produced amongst others two strains of purebred nice, 

 each one lacking a great number of genitic factors. It need hardly 

 be said that these strains were of the utmost utility for a rapid 

 germinal analysis of any individual. 



The material. 



In the course of the experiments I have several times imported 

 mice from different sources. I began the experiments with some 

 albinos, two wild caught females and three black and white ones 

 imported from England. Later I imported a yellow and white couple, 

 reproduced four young like themselves, but were killed with the whole 

 family by a severe frost. My first yellow mice came a yellow female, 

 caught wild in Assen, Holland, long before I distributed any coloured 

 mice there. From this yeUow females I bred the mice of whom I 

 wrote in a preliminary communication. The numerical evidence for 

 the views set out in that paper was however absolutely insufficient. 

 Afterwards another yellow female was produced from homozygous 

 wildcoloured parents, which probable mutation I will describe at 

 some length. 



Two other mutations, each time the spontaneous loss of one 

 genetic factor from a gamete produced by a homozygote containing 

 it, were observed, silvered mice resulting from one of them. 



One brown male, which I bought from a fancier whom I had 

 given some coloured mice a year or so earlier, was found to lack a 

 factor present in all the other mice, and with this one began the 

 fade coloured mice in my cultures. Twice I bought waltzing mice, 

 once pink-eyed agouti ones from Utrecht, Holland, the second time 

 in America. 



There, the mice had to be sent four or five days by rail, so it 

 was impossible to send waltzers, Miss Lathrop, from whom I got 

 them, kindly bred a waltzing male to two females, and sent me the 

 resulting families, from which I bred my waltzers. 



It is because of these different sources from which I got my 

 material, that I was so lucky as to get to know a long list of genetic 



