History of the Tlieory of Heredity. 71 



Comparative Eiiihryology, is that each sexual element 

 originally contains a male portion and a female portion; 

 the ripe male cell is the male half of the male element, 

 and the '^polar globules" contain the male substance of 

 the ovum, which is discharged in order that it may be 

 replaced by the male element from the body of another 

 organism. Balfour says: ^' I would suggest that in the 

 formation of the polar cells part of the constituents of 

 the germinal vesicle, which are requisite for its functions 

 as a complete and independent nucleus, is removed to 

 make room for the supply of the necessary parts to it 

 again by the spermatic nucleus. My view amounts to 

 the following, viz., that after the formation of the 

 polar cells the remainder of the germinal vesicle within 

 the ovum (the female pronucleus) is incapable of further 

 development without the addition of the nuclear part 

 of the male element (spermatozoon), and that if polar 

 cells were not formed parthenogenesis miglit normally 

 occur. A strong support for this hypothesis would be 

 afforded were it to be definitely established that a polar 

 body is not formed in the arthropoda and rotifera ; 

 since the normal occurrence of parthenogenesis is con- 

 fined to these two groups in which polar bodies have 

 not so far been satisfactorily observed. ... To the 

 suggestion already made with reference to the func- 

 tion of the polar cells, I will venture to add the further 

 one, that the function of forming polar cells has been 

 acquired by the ovum for the express purpose of pre- 

 venting parthenogenesis. . . . There can be little 

 doubt that the ovum is potentially capable of developing, 

 by itself, into a fresh individual, and therefore, unless 

 the absence of sexual differentiation was very injurious 

 to the vigor of the progeny, parthenogenesis would most 

 certainly be a very constant occurrence ; and, on the 



