70 Heredity. 



thus acted in many groups of animals ^liere tlie eggs 

 ordinarily require to be fertilized. 



Certain authors have suggested that there may be a 

 connection between the extrusion of the *^ polar 

 globules" from the ovum and the need of impregnation 

 by a male cell. 



The ripe ovarian ovum of an animal usually contains 

 a transparent central body, the germinative vesicle, and 

 when the ^gg is fully ripe the germinative vesicle ap- 

 proaches the surface and divides into two portions : one 

 of these is discharged from the ^gg, thus forming the 

 ** polar globules." These take no jiart in the formation 

 of the embryo. They become entirely separated from 

 the Qgg, and soon die and disappear. The remainder of 

 the germinative vesicle remains in the ^gg, as the "fe- 

 male pronucleus," which unites with the '^male pronu- 

 cleus" formed from the male cell after impregnation, 

 and thus builds up a compound body, the first "seg- 

 mentation nucleus." 



The formation of these " polar globules" has been 

 observed in all groups of the animal kingdom, except 

 the rotifera and arthropods, and their functional sig- 

 nificance is therefore a subject of the greatest interest. 

 They obviously contain something which is not needed 

 for the formation of the embryo, and they may be dis- 

 charged from the ^gg before it is laid, or they nniy re- 

 main until it is laid, as seems to be the general rule, 

 and may be discharged just before fertilization takes 

 place, as is the case in the star-fish, or they muy be dis- 

 charged immediately after the ^gg is impregnated. 



Within recent years an hypothesis regarding their sig- 

 nificance has excited considerable notice. This hypoth- 

 esis, which was first advanced by the late Prof. McCrady, 

 and wdiich is stated at length in Balfour's Treatise on 



