474 



THE AMERICAN NATURALIST 



[Vol. LIV 



no dorsal white may be characterized by an extremely 

 small amount of ventral white. Ordinarily mice with less 

 than 12 per cent, of ventral white may be classed as self. 

 I mated together two such apparently self animals out 

 of a cross of Type "A" by piebald. Each had a small 

 white spot on the belly only covering 4 per cent, of that 

 surface in one of the mice and 5 per cent, in the other. 

 They produced two litters, each of which contained two 

 black-eyed white mice. They were Type "A" mice and 

 not selfs as they had been recorded. A similar case has 

 been reported by Little (1917) in which eleven yellow 

 mice with 6 per cent, and more of ventral spotting proved 

 on breeding tests to be Type "A". That an extreme of 

 Type "A" spotting approached the self condition so 

 closely was not discovered until most of the animals had 

 been graded. It is possible, therefore, that where Type 

 "A" and self animals appear in the same distribution, 

 the self class may be factitiously enlarged at the expense 

 of the Type "A" class, due to errors in grading. Type 

 1 'A" spotted mice are hence indistinguishable somatically 

 from piebalds, and in certain cases, from selfs, although 

 possessing a genetic constitution entirely different from 

 either of these latter forms. 



There remains one other type of spotting to be dis- 

 cussed which like Type "A" and piebald has a different 

 genetic constitution, but which is difficult to distinguish 

 from either Type "A" or piebald. Such mice are pro- 

 duced when Type "A" animals are interbred. Each 

 Type "A" produces gametes WS, Ws, ivS and ws which 

 by random union give the following array of zygotes : 



4 pure for W which are non-viable and die in utero. 



4 WwSs-Tyve "A" 



Type"C" 



2 Wwss— black-eyed white 



2 WwSS-dark spotted (Type "C") 



