376 
GENETICS: A. M. BANT A 
intermediate condition serves to make the sex array in Simocephalus very ex- 
tensive indeed. In fact a precise classification is quite impossible and only an 
extremely relative classification is at all feasible. 
Since the origin of sex intergrades in line 740, the sex intergrade strains 
have descended for nearly three years (about 57 generations). The character 
of the stock seems unchanged — the sex array and proportions of the various 
apparently pure sex forms and intergrades of different sorts continues as it 
was when the intergrade stock first appeared. 
The sex intergrades in Daphnia longispina were first observed in May, 
1917. They appeared in one of the three strains of line 768. This strain 
was not passing through a period of unfavorable conditions at that time nor 
was there anything unusual observed in its behavior except the occurrence of 
sex intergrades. It is indeed not certain that intergrades may not have 
occurred sparingly for a few generations before they were discovered. The 
first intergrade mothers noted in this strain produced many sex intergrades 
among their offspring. Several intergrade strains were at once isolated from 
this stock and placed under observation. 
The secondary sex characters in Daphnia longispina are as follows: (1) Body 
size — the females are larger than the males; (2) Character of the head outline — 
the ventral margin of the head in the female slopes almost uniformly to the 
tip of a pointed rostrum or beak while in the male the beak is absent; (3) and 
(4) Characters of the first {rudimentary) antennae — the paired female first 
antennae are represented only by slight eminences from which arise a few 
sensory stylets, while in the male the first antennae are movable structures 
three-fifths as long as the head. At its distal end each male antenna is armed 
with a stout bristle in addition to the sensory stylets possessed by the 
female; (5) and (6) Character of the breast margins — the female ventral carapace 
margins are almost uniformly rounded from the anterior portion backward. 
In the male these margins are angulated almost to 90 degrees in the region 
nearest the head. Further in the region of this angle these margins are 
fringed with hairs in the male while none occur in the female; (7) and (8) 
Character of the first legs — in the female the terminal joint of each of the first 
pair of legs is not armed with a hook, while this segment in the male bears a 
stout hook. 
That these characters are distinctive of the two sexes is shown by the 
fact that they occur practically without deviation in wild stock, and in fe- 
males 6 and males of the stock not producing intergrades, as well as in the ex- 
tremes of the series in the sex intergrade strains. 
The array of sex intergrades in Daphnia longispina is less extensive than in 
the sex intergrade strains of Simocephalus vetulus in that male sex inter- 
grades (i.e., intergrades with testes) are almost or quite lacking and further 
that males are extremely scarce in the sex intergrade strains of this species. 
That is to say that toward the male end of the series where should occur 
numbers of male intergrades and ostensibly normal males — if the sex array 
