GENETICS: A. M. BANT A 
579 
ing a large percentage of the population. The significance of this ma- 
terial may better be considered after some acquaintance with it. 
About four years ago two lines of a species of small phyllopod crus- 
tacean, Simocephalus vetulus, were taken from outdoor ponds, and their 
propagation continued in the laboratory. Throughout the experi- 
ment young have been isolated in individual bottles when not more 
than a day old. They reproduce parthenogenetically beginning when 
seven to twelve days of age, the first brood ordinarily containing from 
8 to 20 young. 
In addition to numerous other lines of this and other species of Cla- 
docera, six separate strains of one line, '740,' have been propagated. 
For 130 generations these six strains (like all the other strains of Simo- 
cephalus to date) produced nothing but parthenogenetic females. Sev- 
eral lines of each of four other species of Cladocera have been propa- 
gated for long periods of time in the same manner as Simocephalus and 
except for a few males in one of the many lines they have produced 
only females. 
But last October in the 131st generation of one of the strains of line 
740 there suddenly appeared a large percentage of males together with 
normal females and a large number of sex intergrades — males with 
one or more female secondary sex characters, females with one to sev- 
eral male characters and some hermaphrodites with various combina- 
tions of male and female secondary sex characters. 
At this point we may consider the criteria of sex in this species. 
The character of the sex gland, — whether a testis or an ovary — is 
readily determined by microscopic observation of the living animal. 
In addition, eight secondary sex characters distinguish the female from 
the male sex: (1) body size, the females rapidly outstrip the males in 
growth and are considerably larger at sexual maturity; (2) size and 
position of eye, the eye in the female is smaller than in the male 
and does not crowd the margin of the head as in the male; (3) outline 
of the head, the antero-ventral margin of the head is less angulated 
in the female than in the male; (4) absence of the nuchal protuberance y 
there is a slight dorsal protuberance immediately anterior to the cer- 
vical suture, typically in the male only; (5) and (6) character of the 
first {rudimentary) antennae, in the female the basal portion of the 
antenna is not enlarged and bears a single lateral spine (or stylet), 
whereas in the male the swollen condition of the basal portion of the 
antenna and the presence of two lateral stylets are distinctive. Since 
the two antennae in the same individual frequently are unlike, the 
character of each antenna counts as a separate diagnostic character; 
