452 H. J. MULLER 
The fertilized egg, accordingly, must have contained the orig- 
inal unmutated ‘gene for red, and the mutation to white must 
have occurred later in one of the very early ‘cleavage nuclel’— 
perhaps in the two-cell stage—in that nucleus destined to give 
rise to the right eye and to the germ cells. Certainly the muta- 
tion occurred later than the splitting of the chromosomes of the 
one-cell stage and earlier than the separation of the germ tract 
from the ectodermal anlage. 
B. Genetic behavior 
The new white was proved to be identical with the original 
white both in locus and in mode of expression, for when the 
new-white males were crossed to females of the original white 
stock, nothing but whites, indistinguishable from those of either 
parent stock, were produced, either in F;, F:, or subsequent 
generations. Had the loci of the two whites been different, 
some red-eyed crossovers would have been found in F,. More- 
over, the new white, when crossed to eosin, gave a ‘compound’ 
white-eosin of a type exactly like that which eosin gives on being 
crossed to the original white. 
IV. A POSSIBLE ADDITIONAL ALLELOMORPH 
In 1913 the author found, among the offspring of a ATF female 
crossed by f male (these are all recessive sex-linked genes, m 
representing miniature wings and f forked bristles), a single male 
having both eyes of an orange color greatly resembling the 
darker allelomorphs (coral, blood, and cherry) of the W series. 
The shade was intermediate between blood and cherry. The 
male also had the character forked bristles, so that it must 
have been derived from the cross. Its wings were long (not m). 
they may be genetically like either the abnormal or the normal portion. This . 
shows 1) that germ nuclei become divided off from epidermal nuclei only after 
nuclei destined for various parts of the epidermis have become separated from 
each other and, 2) that these germ nuclei (of both sides of the body) are all 
derived from just one such common epidermal-germ-nucleus, and not from two or 
more independently. 
