28 HEREDITARY CHARACTERS 



fertilised ovum receive half their chromosomes from each 

 parent. 



There are some cases, however, among multicellular plants 

 and animals, where new individuals are produced from a 

 single cell derived from one individual without fertilisation 

 taking place. Such individuals have one parent only, which 

 produces them without the intervention of a second parent 

 as is usually necessary. This phenomenon is known as 

 "parthenogenesis." In parthenogenesis. both among animals 

 and plants, no fertilisation takes place. For instance the 

 female Aphis (common green-fly) produces other females 

 without any intervention of the male. These females again 

 produce females, and so on for many generations throughout 

 the summer, without any fertilisation taking place. The 

 same thing happens in the case of bees. The queen-bee lays 

 a number of eggs, some of which are fertilised, and some of 

 which are not. Those which are unfertilised develop into 

 drones ; those which are fertilised develop into queens or 

 workers, according to the manner in which the larvae are 

 fed. There are many other cases of parthenogenesis among 

 animals and plants, and in some of them it has been demon- 

 strated that one of the polar bodies, after having been thrown 

 off, re-enters the ovum, and acting as a sperm cell, brings 

 back the number of chromosomes to that normal to the species. 

 In other cases, reduction apparently does not take place. 



In many of these parthenogenetic organisms, although 

 a number of generations of females are produced without 

 fertilisation taking place, males appear at different periods 

 and a generation is produced from fertilised eggs. Thus in 

 the case of Aphis, towards the end of the summer some males 

 are produced, and the ova are fertilised in the next genera- 

 tion, producing the so-called " winter eggs." In this case the 

 phenomenon of fertilisation recurs regularly at the end of 

 every summer. There are many other similar cases, but 

 there are others again among animals, and more still among 

 plants, where fertilisation is extremely rare or never takes 

 place at all. 



