22 



ZOOLOGY 



SECT 



is a normal phenomenon in certain families of insects, for examplr 

 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- 



pol 



B 



went 



rrwnv 



seg nucL 



FIG. 7. Diagram illustrating the maturation and fertilisation of the ovum. A, formation of 

 first polar body; B, beginning of fertilisation, sperms approaching the micropyle ; (', 

 formation of the male pronucleus ; D, approximation of the male and female pronuclei ; 

 E, formation of segmentation-nucleus ; ? cent, female centrosome ; <$ cent, male centro- 

 some ; mem. egg-membrane ; microp. micropyle ; pol. polar bodies ; 9 pron. female 

 pronucleus ; pron. male pronucleus ; 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 



