CHAPTER IX 



Parthenogenesis 



TTN the great majority of animals reproduction is bi- 

 JL parental new individuals arise from" a zygote formed 

 by the union of an egg and a spermatozoon. There are, 

 however, at least three other methods of reproduction 

 which are widespread among the lower animals and found 

 to some extent even in some of the "higher." These are 

 fission, budding, and parthenogenesis. Fission is of course 

 a common method of reproduction among the Protozoa, 

 but is not by any means confined to them ; it occurs in the 

 adults of some Coelenterates and Echinoderms, and em- 

 bryonic fission is found in certain Polyzoa and Insects. 

 Budding is not quite easy to separate sharply from fission; 

 they grade into one another, though usually the two pro- 

 cesses are sufficiently distinct. Parthenogenesis belongs to 

 a rather different category, though at times it is possible 

 to confuse it with budding. It may be defined as the pro- 

 duction of a new organism from an unfertilised egg, and if 

 a bud should arise from a single cell, it is evident that the 

 decision as to whether the process is or is not parthenogene- 

 sis would depend on whether the cell were or were not an 

 egg. Usually there is no difficulty; in a few cases, as in the 

 paifdogenesis of the Cecidomyid flies, some observers have 

 maintained that what is in fact true parthenogenesis is 

 internal budding. 



True parthenogenesis occurs in nature under a number of 

 different circumstances. Most frequently it alternates more 

 or less regularly with bisexual reproduction; in only very 

 few species is it the only method of reproduction known. 



