SEX INHERITANCE 97 
living male it must be assumed that the no-X 
sperm is non-functional. The X-bearing sperms 
would fertilize the eggs and produce the hermaphro- 
ditic females. 
PARTHENOGENESIS AND Sex 
In the animal kingdom there are several cases 
where a species reproduces for several generations 
by parthenogenesis, and, then, at the end of the 
chain, sexual forms appear. The rotifer, Hydatina 
senta, is the best illustration since the change can 
now be controlled (Whitney). This animal ordi- 
narily produces parthenogenetic eggs (Fig. 33, 
A, D), which give rise to females. If fed on a pure 
diet of the protozoon, Polytoma, only these par- 
thenogenetic females are produced in successive 
generations (Whitney, Shull). If the diet is changed 
to the green alga (Euglena) a female will then lay 
eggs that give rise to a new kind of individual—a 
male-producing female which is not externally 
different from its mother. If the male-producing 
female is not fertilized by a male, soon after hatch- 
ing (B), she produces a large number of small eggs 
(E) that develop parthenogenetically into males 
(C), but.if she is fertilized she produces a few large 
eggs with thick shells (F)—the resting eggs—that 
always become females. Hence the change in diet 
has caused the appearance of a new kind of in- 
dividual that functions as a sexual female with the 
production of a few, daughters, or as a partheno- 
