37 2 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. 



Partheno genetic 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 1 

 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 



1 Maupas records seven parthenogenetic species in the Nematodes. 



