372 Experimental Zoology 
by the series of species showing intermediate stages. Incomplete 
females are found in which one ovary alone produces both kinds 
of germ-cells, the other only the female germ-cells, and still 
other individuals are hermaphroditic in both ovaries. 
Parthenogenetic Species 
Parthenogenesis is also of not infrequent occurrence in the 
animal kingdom. It is found most often in insects and in the 
lower crustaceans and rotifers. It is found in some Nematodes ' 
and in a few other groups. In many cases the eggs of unisexual 
forms may begin their development if not fertilized, and in recent 
years it has been shown that parthenogenetic development may 
be induced artificially in many species of animals. Thus we 
have come to look upon the egg as capable of producing an em- 
bryo if the proper external stimulus to development is present. 
In nature this process has been carried out quite often with 
far greater success than has been as yet accomplished by arti- 
ficial means. 
In some cases, as in the aphids, the parthenogenetic females 
(wingless or winged) can be distinguished structurally from the 
egg-laying females. The latter alone have the receptaculum 
seminalis for storing the sperm. In other cases the two 
kinds of females may be, externally at least, almost identical. 
The ova are, however, sometimes different, as in daphnia. . 
Since the eggs of so many species show incipient parthenogene- 
sis, we have come to regard natural parthenogenesis as having 
arisen by the disappearance of the males in certain generations. 
The rareness of the males in certain groups in which partheno- 
genesis has replaced the sexual modes of reproduction — in some 
rotifers, sawflies, etc. —seems to support this point of view. 
The question is, however, not quite so simple as this if we exam- 
ine it more closely. What has become of the male eggs if this 
view is correct? Do they simply fail to. develop, or have they 
become female eggs? There is no evidence to show that half 
* Maupas records seven parthenogenetic species in the Nematodes. 
