138 PARTHENOGENESIS [en. 



widely separated from it in classification, the egg may be 

 caused to develop although the sperm nucleus, or its 

 chromosomes, take no part in development. This has been 

 observed, for example, in the eggs of Echinoids fertilised 

 by sperm of the Mussel (Mytilus) 1 , and cytologically the 

 development must be regarded as parthenogenetic, although 

 a spermatozoon is present and has stimulated the egg to 

 develop. It has more recently been discovered that a 

 similar process takes place naturally in at least one Nema- 

 tode worm Rhabditis aberrans^ described by EVA KRUGER 

 (1913). In this species the vast majority of specimens are 

 protandrous hermaphrodites, which produce spermatozoa 

 early in the stage of sexual maturity, after which only eggs 

 are produced. Very rarely true males occur. When the eggs 

 are ripe, spermatozoa stored up from the earlier sperm- 

 producing stage enter them, but take no part in develop- 

 ment, and very soon degenerate. Only one polar body is 

 formed, and in the polar mitosis the chromosomes appear 

 in the somatic number and divide equationally. In a single 

 egg the formation of a second polar body was observed, and 

 in this case the egg nucleus conjugated with the sperm 

 nucleus. It is suggested that the exceptional males are 

 produced from these rare fertilised eggs. 



The chief cytological facts of natural parthenogenesis 

 may be summarised as follows : 



1. Perhaps the most frequent type is that in which the 

 egg nucleus undergoes one maturation division and no 

 chromosome reduction, so that the single polar nucleus and 

 the egg nucleus both contain the diploid number of chromo- 

 somes. This is characteristic of Crustacea, Homoptera, 

 Cecidomyidae, and some Rotifers. 



2. In rare cases (e.g. Artemia, Xenos) there may be two 



1 KUPELWIESER (1909, 1912) 



