The Genetic Factors in tht- Dovelopni nt of the Housemouse, etc. 12 1 



that in the rabbit there have not yet been made very many com- 

 binations of factors without c. It has been my good fortune to 

 witness the probable loss of tlie factor B from a mouse-germ, this 

 factor being hitherto present in all mice, and absent only from certain 

 rabbits and cavies. 



On the other hand it has been possible in the rabbit and the 

 cavy to study a few factors which are either not present in the mouse, 

 or else present in all individuals. I will here give some of such factors, 

 which probably will be some day known in the mouse as well, though 

 they are not known as yet in that animal. 



Factor M. 



This factor is inherited quite independently from the others. Its 

 presence is indispensable for a full action of the factor B. The factor 

 M has been found to be absent in certain cavies and certain rabbits 

 (and cats), but never in mice. 



In those individuals having B in their formula, absence of M 

 makes that the black or brown colour resulting from B with other 

 factors is restricted to part of the coat, the rest remaining yellow. 

 Thus are ABCDEFgHM animals black, and ABCDEFgHm ones are marked 

 black and yellow. ABCDEFGHm animals are agouti and yellow, the 

 yellow being of the sort found in yellow agoutis, abcdefghm being 

 solid agouti. Absence of D from such animals does not affect only 

 the darker parts, but the whole coat, so that ABCdEFgHm becomes 

 blue on a Hght yellow (cream) background. If such marked animals 

 are without L, they have white markings, and these are placed in- 

 dependantly from the distribution of the darker spots on the yellow 

 background. Such animals are called tricolour. They are very common 

 in cavies and rabbits. The colours may be chocolate, orange and 

 white, or agouti, yellow and white or black, yellow and white. 

 Tricolour mice can not be produced unless at some time there are 

 by mutation produced mice without M, or if it will be possible to 

 breed fertile hybrids between mouse and cavy or mouse and rabbit. 

 Castle has studied this factor m and proved its independance from 

 factors B, c and L. 



Factor N. 

 Another factor known in rabbits and cavies but not in mice is 

 that which is present in aW albino (and therefore undoubtedly also 

 in all coloured) cavies and in some albino rabbits. Its presence causes 



