132 WILLIAM B. KIRKHAM 
appear in these specimens, which offers some additional support 
to the statement of Asai ('14) that this type of cell is embryonic 
in origin. 
The similarity in the histological details of the absorbed em- 
bryos in white and in yellow mice might be taken to prove the 
identity of the underlying causes in the two cases, but the as- 
sociated facts tend to modify any such view. The degenerate 
embryos from white females were obtained, with possibly one 
exception, from animals which were more or less pathological, 
and the one possible exception is quite likely not such, as the 
preceding litter in that instance was removed at birth and other- 
wise might not have survived for long. Thus in white mice 
the absorbed embryos might all be accounted for on the basis 
of pathologic uterine environment which selectively disposed of 
the weaker members of the sets of blastulas, the set mates sur- 
viving. The same factor may be present in all yellow females, 
but in these animals, instead of the unfavorable environment 
being abnormal, we should have to assume, on account of the 
universality of the phenomenon, that it is actually a normal 
correlation with yellow coat color, an assumption further sup- 
ported by the marked tendency in yellow mice of both sexes to 
fatness and sterility at a relatively early age. This matter will 
be subjected to further investigation through a projected study 
of the non-yellow" offspring from yellow matings which, if the 
above assumption is correct, should be differentiated in general 
vitality from control animals, offspring from non-yellow^ parents. 
Parental abnormality, however, cannot account for all the 
facts connected with the failure of homozygous yellow mice to 
be born, and we must further assume an inherent weakness, or 
lethal factor, in all the homozygous yellow embryos which 
invariably brings about their destruction during implantation, 
while their fellows in the same ovulation and environment, but 
endowed with factors associated with a different coat color, im- 
plant and develop normally. 
The assumption is warranted that the degenerate embryos 
found regularly in pregnant yellow mice mated with males of the 
same coat color are the missing homozygous animals, for their 
