TWO KINDS OF GERM-CELLS 489 



of ova one kind constitutionally predestined to developing 

 into males, the other kind constitutionally predestined to de- 

 veloping into females. This view is not inconsistent with the 

 assumption, which seems almost inevitable, that all ova carry 

 a complete hereditary equipment of both masculine and feminine 

 characters, though only one set usually finds expression. But 

 what evidence is there of two kinds of ova ? 



Some animals normally produce two sizes of ova. Thus, in 

 Phylloxera among insects, and Hydatina senta among Rotifers, 

 there are large eggs which develop into females, and small 

 ones which develop into males. As both develop without 

 fertilisation, the problem is not complicated by the influence 

 of the sperm. 



In Dinophilus apatris, according to Von Malsen, and in a mite, 

 Pediculopsis , according to Reuter, where fertilisation occurs 

 as usual, there are large ova which develop into females and 

 small ova which develop into males. In Dinophilus the ovum 

 which becomes a male is only about one-tenth of the size of 

 that which becomes a female ; and the male himself is a de- 

 generate pigmy ! 



Perhaps the occurrence of two sizes of ovum is much commoner 

 than we know. Thus Baltzer has recently described it in sea- 

 urchins. But we must not hastily assume that it is the size that 

 determines the sex, since it may be that the constitutional pre- 

 disposition to one sex or the other determines the size. On 

 our own theory, the ovum of relatively greater anabolic bias 

 predestined to develop into a female will tend to gather into 

 itself more reserve material than one predestined to develop 

 into a male. It seems probable that the size marks, but does 

 not make, the difference. 



Of great importance is the work of Riddle (see p. 618), showing 

 that in pigeons there are eggs with high storage metabolism 

 which develop into females, and eggs with a low storage capacity 

 which develop ji)to rnalos, 



