AN ANDROGYNOUS DAPHNIA PULEX FROM WARLEY. 87
Although there is every reason to believe that sexual
characters, like all other heritable characters, ultimately depend
upon particular elements, genes, in the chromosomes, the way
in which they exert their control is still far from being thoroughly
understood. Especially is this so in such cases as the Cladocera,
the group to which Daphnia belongs, in which the great
majority of the individuals, often for generation after generation,
are parthenogenetic females. It has been experimentally
shown2 that in this group the production of fully-sexed or
gamogenetic females (also referred to as ephippial females)
and males is somehow initiated by the influence of some factor
or combination of factors in the external environment and
further that the same conditions tend to the production of both
at the same time, although there is no causal connection between
them.
It was further shown by Mortimer3 that all eggs produced
by parthenogenetic females are "diploid," their ripening
process not including the halving of the number of chromosomes
during the maturation divisions observed in eggs of animals
which reproduce bisexually. They can develop without
fertilization and the resulting offspring possess the same diploid
chromosome number as the mother. No obvious difference was
found either in chromosome number, shape or size which would
explain why some parthenogenetic eggs develop into females
and others into males. As, however, in the case of the male,
distinguishing characters of this sex can be recognised quite
early in embryonic development in the brood cavity, it appears
probable that the external influences which determine the sex
of the male individual do so at a still earlier stage, possibly
during the development of the egg in the ovary. The mechanism
of this determination is, however, at present unknown and it is
therefore of great importance that, not only normal cases but
also exceptional ones, such as that described above, of an
individual with both male and female characters should be taken
into account in any attempt to unravel this problem.
Although, as already mentioned, there is no causal connection
between the production of males from parthenogenetic eggs
2 See C. H. Mortimer, "Experimentelle und cytologische Untersuchungen uber der
Generationswechsel der Cladoceren." Zool. Jahrbucher—Abt. f. allg. Zool. Band 56, Heft 3, 1936,
pp. 323-388.
3 loc. cit.