13 



Amphimixis is "a mingling of different hereditary ten- 

 dencies in one and the same individual." It is the result 

 of fertilisation, of the fusion of a male and a female pro- 

 nucleus. 



What is Parthenogenesis ? 



In some exceptional instances individuals are developed 

 from ova which have not been fertilised by sperms. A 

 good example is that of the male or drone hive bee which 

 arises from an unfertilised egg. This phenomenon also 

 occurs among Rotifers, Aphides, and certain Crustaceans. 

 In the malaria parasite (Plasmodium vivax) of tertian ague 

 the female gametocytes are believed by some to reproduce 

 by a sort of parthenogenesis, and in this way give rise to 

 fresh schizogonous generations. 



Explain what is meant by Metagenesis or " Alternation of 

 Generations." Describe the Life-cycle of any Animal 

 which exhibits this Phenomenon. 



In the life- history of certain animals reproduction is 

 asexual at one time and sexual at another, asexual genera- 

 tions (there may be several) intervene between sexual genera- 

 tions. That is " Alternation of generations " or Metagenesis, 

 which has been defined as " the alternate occurrence in 

 one life-cycle of two (or more) differently-formed organisms 

 differently produced." It is seen in the life- histories of the 

 Malaria parasites and other Haemosporidia, where several 

 asexual generations (by schizogony) alternate with sexual 

 generations (by sporogony). It is also characteristic of 

 the life-cycles of Polystomella, Spongilla, Hydroids (e.g., 

 Obelia), Aurelia, Distomum syn. Fasciola (the Liver Fluke), 

 Tunicates, etc. In those instances (e.g., Distomum and 

 certain Nematodes) in which two different sexual genera- 

 tions (e.g., a par^henogenetic and a true sexual) alternate, 

 it is known as heterogamy. 



NOTE. For answer to second part of question see de- 

 scriptions of the above-mentioned life-histories. 



