A Comparative Study of the Chromosomes etc. 
279 
may be found. Miss Stevens describes the Separation of the conjugants 
in the strepsistene stage as taking place by a process of unroUing. So 
far as regards ^ncÄenopa, the threads are so indistinct in the late strepsis- 
tene stage that no evidence could be gathered either for or against syn- 
niixis during this stage. 
The reasons for believing that the hetero-hoineotypic niethod of 
division takes place in Enchenofa have been given in the description 
(p. 271—272, 274), the evidence being based mainly upon the deport- 
nient of the macrochromosomes. In niost of the recent investigations of 
the spermatocyte divisions in numerous insects, this is the type that has 
been found to exist. Gregoire (’05, ’IO) has given a complete review 
of these cases, as weil as of those which differ froni this type, namely, 
those described by McClung (’05), Gross (’04a, ’04b, ’06), Otte (’06), 
and Wirke C07). 
B. The Sex-Chromosomes. 
During the past several years niuch has been done to extend our 
knowledge of the so called sex-chromosomes. A complete history of 
their behavior in the matnration divisions in both sexes has been foUowed 
in the insects (Morrill ’IO) and in the nematodes (Gulick ’ll), and the 
results agree perfectly with the hypothesis first brought out by Stevens 
and Wilson: namely, that the female is homozygous in regard to sex 
chromosomes (2a:), while the male soma is heterozygous {x) or (x+y)’, 
that (in general) all the matured eggs have the sanie chromosomal Con- 
stitution, but that the spermatozoa are of two kinds, those with an x~ 
chromosome, “female determining spermatozoa”, and those without 
an a>-chromosome (or with a ^-chromosome in place of an x-chromosome) 
“male determining spermatozoa”. It has also been shown that there is 
a definite association of these heterochromosomes (assuming that they 
are not the primary determinants of sex, but nierely an index of sex) 
in the case of the sexual and hermaphrodite generations of the nematode 
Angiostomum (Schleif, ’lla, ’llb; Boveri, ’ll), likewise in partheno- 
genetic and sexual generations of the aphids and phyUoxerans (Morgan 
’08, ’09; VON Baehr ’09), and in the hybridization of the ($) guinea 
fowl with the (cT) common fowl (Guyer ’12). The facts brought out by 
these researches also lend Support to the hypothesis that, in these cases 
at least, the male normally produces two sorts of spermatozoa and that 
in those cases where only one sex results from sexual reproduction, one 
of the two classes of spermatozoa either fails to develop or is non-func- 
tional. 
