The Sex Chromosomes. 253 
is odd in some insects of this type had previously been determined 
by several observers (Sinety, Montgomery, Sutton, 
Me Clung); but its significance was obscured because some 
observers had described the number as even in other species of 
the same type (Henking, Paulmierand even Montgomery) 
while practically nothing was known of the conditions in the 
female. 
In the mean time (1905a) I discovered a second type of 
dimorphism of the spermatozoa in several genera of Hemiptera 
(Lygaeus, Coenus, Euschistus, Brochymena); and a similar type 
was independently found at the same time by Miss Stevens in 
one of the Coleoptera (Tenebrio). In these forms the male 
contains in addition to the X-chromosome a characteristic smaller 
chromosome which I called the "small idiochromosome‘“, later the 
”Y-chromosome“ (1909 ce). In maturation, X and Y play the part 
of synaptic mates, and pass to opposite poles in one spermatocyte- 
division, so that half the spermatozoa receive X and half Y. 
Stevens (1905), followed by myself (1905 b, ce, 1906) discovered 
that in this type, the female contains two X’s in place of X and Y. 
The two sexes, accordingly, contain the same total number of 
chromosomes, but nevertheless differ as characteristically as in 
the first type, the female composition being XX and the male 
XY. There is here double evidence that males or females are 
produced according to the kind of spermatozoon that enters the egg. 
For, on the one hand, the Y-chromosome, readily distinguishable 
by its small size, must be derived from the Y-elass of sperma- 
tozoon, and it is confined to the males. On the other hand, the 
relations of the X-chromosomes are the same as in the first type, 
and justify the corresponding conelusion regarding the X-class of 
spermatozoa. 
We may therefore accept the following formulas for sex- 
production in the two respective types of these animals: 
a) Egg X + Spermatozoon X =XX (Female) 
b) Egg X + Spermatozoon noX= X (Male) 
ne Dips a) Egg _ er eu) x T AX (Female) 
ı b) Egg X + Spermatozoon Y = XY (Male). 
lf Y be supposed to disappear, the second type becomes 
identical with the first. It is almost certain that such has been 
( 
I. Protenor Type | 
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