104 Hagedoorn. 



Factor A. 



This factor is indispensable for pigmentformation, all the animals 

 without it are necessarily albinos, and all coloured animals contain 

 it. It is therefore a very easily studied factor, and I believe that 

 every author on the subject knows it as such. It is not only in mice 

 that this factor sometimes fails, but there are also individuals of the 

 brown rat, the cavy, the rabbit, the hamster, which seem to differ 

 from coloured animals because of a lack of this same factor. It is 

 of course impossible to say, that in all these animals this factor, 

 distinguishing by its presence or absence coloured and albino animals 

 is identical. To make sure of this, we would have to cross some of 

 these albinos, and find that the hybrids would also be albino. This 

 has been done by Iwanoff for the brown rat and the mouse, a 

 female albino Mtis musculus, artificially inseminated with sperm of 

 Mus decumamis gave two albino young. Neither in the mouse nor in 

 the brown rat do there exist two different factors, indispensable for 

 pigmentformation, for otherwise the mating of two albinos would 

 occasionally produce coloured offspring (as in the case of Lathyrus). 

 I have from my records added the number of coloured and albino 

 young produced in thos matings where an animal heterozygous for 

 A, (an Aa . . .) was mated to an albino. Such matings were very often 

 made, to find out whether any given animal was homozygous or 

 heterozygous for A. 



In counting together the young produced from such matings I get : 



340 coloured (Aa . . .) young and 364 albino {aa . . .) young, the 

 calculated ratio being 352 : 352. 



Matings between heterozygous coloured (Aa . . .) and albino (aa . . .) 

 rabbits have been made by Castle and Hurst, for the brown rat, 

 the ratio of albino and coloured young produced in six matings of 

 Aa with albino (aa) animals in my experiments was 21 coloured (Aa . . .) 

 and 27 albinos (aa . . .), the calculated ratio being 24 : 24. 



The experimental proof for the independance of this factor A from 

 the factors B, c, D and G will be given at the end of this paper. 



All the mice figured on the coloured plate contain A, with the 

 exception of number 2. 



Factor B. 

 This factor I could only study because once it got lost by 

 mutation in my cultures. It seems that none of the other authors 

 who worked with mice have ever been able to study this factor. 



