Ill 



PARTHENOGENESIS 87 



only half the number of chromosomes present in the other tissues of the 

 body. Otherwise this number would be halved in each generation, and 

 very soon brought to vanishing point. This eventuality is avoided by 

 one of two methods. 



(i) The halving of the chromosome number in oogenesis is omitted, 

 and the ripe egg is diploid. This is usually accompanied by the formation 

 of only one polar body instead of two, and is the commonest form 

 of parthenogenesis. It is found in the Cladocera, Ostracoda, Aphids, 

 etc., amongst Arthropods and in Rotifers, etc. Occasionally, however, 

 two polar bodies are formed, without reduction of chromosomes (the 

 Hymenoptera Nematus and Rhodites). 



(2) True meiosis takes place, the resulting ripe egg being haploid, 

 but reduction is omitted when the haploid individual developing from this 

 egg forms its own gametes. This form of parthenogenesis has been 

 found in several Hymenoptera. In all cases so far known, the haploid 

 individual thus produced is a male. This, it will be observed, is in 

 accordance with the fact that in the great majority of animals in which a 

 difference in the chromosomes of the sexes has actually been observed 

 the male lacks the second sex chromosome, or has it replaced by an inert 

 chromosome (Chapter IV.). For it is obvious that a haploid animal 

 can have only one of each kind of chromosome, including the sex 

 chromosome. 



The eggs of type (i) being diploid are incapable of fertilization, and 

 therefore committed to develop parthenogenetically ; they are said to 

 exhibit obligatory parthenogenesis. The haploid eggs of type (2) differ 

 in no way from eggs destined for fertilization. They seem indeed to 

 be equally capable of fertiHzation or parthenogenetic development. If 

 fertiUzed they give rise of course to an ordinary diploid individual, which 

 in all cases known is a female. If they are not fertiUzed they develop 

 parthenogenetically into a haploid male. Parthenogenesis in these cases 

 is therefore said to be faciiUative. 



Further, the eggs of many animals which normally develop only 

 after fertiUzation can be induced to develop parthenogenetically by the 

 application of appropriate stimuli. Such eggs produce either haploid 

 or diploid individuals according as to whether meiosis had or had not 

 taken place before the parthenogenetic development was induced. This 

 phenomenon is known as artificial parthenogenesis. 



The cytology of these three types of parthenogenesis will be considered 

 in order. 



(i) Obligatory Parthenogenesis 



By far the greater number of these cases are accompanied by the 

 formation of only one instead of two polar bodies, and the chief interest 



