588 
GEOFFEEY SMITH. 
lieterozygotes, as one and the same female may give rise par- 
tlienogenetically to males and females. It is also proved in 
these cases that no segregation occurs in the pi'oduction of pai'- 
thenogenetic females, since a parthenogenetically produced 
female may give rise by parthenogenesis to a mixed brood of 
males and females. We may be certain, therefore, that the 
females in these cases are heterozygotes ((J $ ). With regard 
to the parthenog’enetically produced males we are naturally 
more in the dark, since they produce no parthenogenetic 
young by which they can be judged. The most obvious 
supposition is that, since they, like the females, are produced 
parthenogenetically, there is no segregation in their produc- 
tion, and that they also are heterozygous (c?" ? ) and produce 
and ? spermatozoa. If, however, we are going to main- 
tain the half-hybridism theory of sex, since the females are 
certainly ? the males must be pure ^ (5',and some process 
of segregation must occur. But if this is the case why do 
the eggs, when fertilised with such purely male spermatozoa, 
invariably give rise to females ? It has been held, especially 
for the bee, that the mere act of fertilisation in itself is the 
cause of the production of females, and if this is the case it 
is very difficult to bring the phenomenon into any relation 
with Mendelian theory. There is, however, an alternative to 
this explanation. AVe may legitimately hold that the female 
gives rise to two different kinds of eggs, male and female, 
of which only the female egg’s are capable of being fertilised. 
Such female eggs, being fertilised by the male spermatozoa, 
will give rise to heterozygotes of the compo.sition ? , which 
ex hypothesis will appear as females, while the unfertilised 
male eggs will give rise to males of pure male constitution. 
In the Rotifer Hydatina and in the worm DinoiDliilus 
we know that two different kinds of eggs are produced, 
large eggs which give rise to females and small eggs which 
give males, and this fact seems at first to favour the theory 
proposed above. In the case of Dinophilus, however. 
Dr. Shearer, in a recent unpublished research, has shown 
that the female Dinophilus is fertilised while still immature 
