59) A. H. STURTEVANT 
The cytological evidence relating to birds and Lepidoptera 
is not very helpful. Guyer’s (09, ’09 a) reports on guinea-fowls 
and chickens are directly opposed to the experimental evidence, 
in that they make the male heterozygous for sex. Since a re-ex- 
amination of the fowl case by other cytologists has so far failed 
to convince them that Guyer’s view is correct, I think we may 
for the present disregard this evidence, at least in so far as it 
concerns the fowl. Several observers (see Stevens, ’06; Dederer, 
07; Cook, ’10; Doncaster, ’11) have studied the spermatogene- 
sis of Lepidoptera, and in some cases have seen what they sus- 
pected to be an equal pair of idiochromosomes. I do not know 
of any further cytological evidence in these two classes of animals. 
000 GU ub Ya ue 
io of Q of 
2 3 4 
FE 
The cytological evidence indicates that, in the Lepidoptera 
at least, the male has two equal idiochromosomes. Judging 
by the experimental evidence, this must also be the case in birds, 
and the female must, of course, have at least one similar chro- 
mosome (see figs. 2 to 4). These three are the carriers of the genes 
for sex-linked factors. The doubtful point is the mate of this chro- 
mosome, present in the female-producing egg. If the sex formula 
be. Ff, ff, as we have been supposing, then this chromosome 
would be, visibly or imperceptibly, larger than the ‘male’ (f- 
bearing) chromosome, since it would have one factor, F, not 
present in that chromosome (fig. 2). In this case it would seem 
that complete sex-linkage, such as that found in Abraxas and 
in barred fowls, would occur not at all, or at least only rarely, 
since every part of the f-chromosome would have a homologous 
part in the F-chromosome, and crossing over would thus be pos- 
sible. It might be that the process of reduction is such that 
no crossing over is possible in oogenesis, but if the cases of par- 
