494 HEREDITY AND SEX 



Size of the Egg. — There are some suggestive facts in regard 

 to the size of the ovum. It is well known that some animals 

 normally produce two sizes of ^gg, and in certain cases [Phylloxera 

 among insects, Hydatina senta among Rotifers, and Dinophilus 

 apatris among worms) the large eggs produce females and the 

 small ones males. In Dinophilus the large ova which develop 

 into females are said to be opaque, while the small ova which 

 develop into males are said to be transparent. In the phraseo- 

 logy adopted in The Evolution of Sex, the eggs in which the 

 anabolism is relatively high become females, and the eggs in 

 which the anabolism is less high become males. Attempts have 

 been made to extend this generalisation, e.g. to the large and 

 small eggs of the silk-moth, but the results are conflicting. 



Moreover, as Prof. T. H. Morgan points out (1907, p. 396) 

 " even if large eggs produce fewer males and small eggs more 

 males, it is not clear whether the result is due simply to the size 

 determining the sex, or whether the female eggs tend to become 

 larger than the male eggs. If analogy has any value in this 

 instance [moths], it seems more probable, from the cases of 

 Phylloxera, Dinophilus, and Hydatina, that sex may be sometimes 

 predetermined in the egg and this determines its size." 



We may recall the well-established fact that the so-caUed 

 " winter " eggs of Aphides and Cladocera, which are fertilised, 

 always develop into females. 



Two Kinds of Eggs. — It has often been suggested that there 

 may be two kinds of eggs, one kind predisposed towards develop- 

 ing into females, and the other predisposed towards developing 

 into males. In the same way there may be two kinds of sper- 

 matozoa similarly predisposed towards developing into males or 

 developing into females. A fertilisation of an ovum predisposed 

 to female development by a sperm predisposed to female develop- 

 ment would naturally result in a female, and vice versa. And the 

 result of the combination of an ovum predisposed to female 

 development with a spermatozoon predisposed to male develop- 



