POLAR CELLS OF OVUM. 141 



The reproductive organs are a development from the 

 hypoblastic membrane or entoderm. 



A description of the gradual evolution of these 

 organs is far above the scope of this Monograph : the 

 subject is of a nature too special ; and, moreover, it has 

 been adequately treated by others whose memoirs may 

 be advantageously studied in detail. 



Relative to the development of the ovum in Aphis 

 much interesting information (by way of analogy) 

 may be gathered from Prof. Schafer's article on ova 

 in the ' Quart. Journ. Microscopical Science,' and also 

 from another article, in the same number, by Prof. 

 Balfour on development of the Araneina. 



3.-SEX IN APHIS. 



The significance of the polar cells in the ovum is not 

 certainly known. They occur in the unimpregnated 

 egg, and they result from and are coexistent with the 

 breaking up of the germinal vesicle, parts being appa- 

 rently extruded from the ovum. After the formation 

 of the polar cells, it would appear that impregnation 

 only will develop the egg, but possibly before their 

 occurrence parthenogenesis might occur. 



The evils of self-fertilisation, viz. the want of suffi- 

 cient differentiation in the sexual elements, would 

 apply with far greater force to cases of partheno- 

 genesis ; and it has been suggested by Prof. Balfour 

 that this evil is prevented by this very function of 

 forming polar cells.* 



Although Biitschi was unable to see any polar cells 

 in the pseudova of the viviparous Aphis, they may be 

 certainly brought into view by due precaution. 



Yon Siebold's investigations led him to believe that 

 the eggs bring from the ovary the capacity of differen- 

 tiating themselves into living male individuals; and 

 that the same eggs can be changed as to their proper- 



* Balfour, ' Treat. Comp. Embryol.' 



