250 Edmund B. Wilson: 
group (represented by certain air-breathing arthropods, nematodes 
and vertebrates) there are two kinds of spermatozoa and but one 
kind of egg. In a second group (sea-urchins) this relation is 
reversed, there being two kinds of eggs and but one kind of 
spermatozoa. In both groups, accordingly, one sex may be 
characterized as ”digametic“, the other as "homogametiec‘ (Wilson, 
1910a). The relations prove that the two kinds of gametes 
formed in the digametic sex produce, respectively, males and 
females upon union with gametes of the opposite sex. In the 
process of maturation the two kinds of gametes are separated 
by one of the maturation-divisions. They are therefore equal in 
number; and from this follows the general law of the numerical 
equality of the sexes. The review which follows relates primarily 
to those cases in which the male is the digametic sex, which 
alone, with a single exception, have thus far been eytologically 
analyzed. 
The simplest type of sex-chromosome was first discovered 
by Henking (1891) in Pyrrhocoris, where it is in the male a 
er 
\ ERS 
= 
single chromosome which passes undivided to one pole in one of 
the spermatocyte-divisions, and hence enters but half the sperma- 
tozoa. This was first confirmed in my laboratory by Paulmier 
(1399) in Anasa, afterwards by Montgomery in Protenor (1901) 
and by Sindty (1901) Me Clung (1902) and Sutton (1902) in 
