22 



ZOOLOGY 



SECT. 



is a normal phenomenon in certain families of insects, for example. 

 In a considerable number of marine invertebrate animals it has been 

 shown that though gamogenesis, i.e. development as the result of 

 fertilisation of ovum by male cell, is the normal process, yet parthe- 



t>ol 



in em 



menv 



m.Lcro/3 



^ /fro n 



Flo. 7. Diagram illustrating the maturation and fertilisation of the ovum. .-1, formation of 

 first polar body; R, beginning of fertilisation, sperms approaching the mieroiiyle ; (', 

 formation of the male promicleus ; 1), approximation of the male and female proniiHej ; 

 E, formation of segmentation-nucleus; y cent, female cehtrosome : <.', crnt. male eentn*- 

 some ; mem. egg-membrane; microp. micropyle ; pol. polar bodies; y j>rn. female 

 pronucleus ; c$ pron. male pronuoleus ; seg. nucl. segmentation nucleus. 



nogenesis can be produced by various artificial means. By adding 

 various salts to the water in which the ova are contained, by changes 

 of temperature, or by subjection to the action of carbonic acid gas, 

 the ova, in the absence of sperms, may be caused to give rise to 



