22 



SEX AND HEREDITY 



makes it seem probable that some real advantage has 

 prompted it. The advantage appears to lie in the fact 

 that the larger the amount of food that is contained 

 in the egg the better nourished the offspring will be at 

 its first stages, and the better accordingly will be the 

 chance of its passing successfully through the dangerous 



FIG. ii. 



Examples showing increasing difference in proportion of the pairing 

 gametes in the green Siphonocladiales, and Siphonales. (After 

 Oltmanns.) i, isogametes of Acetabularia; ii, unequal gametes of 

 Bryopsis ; iii, unequal gametes of Codiitm ; iv, motile spermatozoids and 

 non-motile egg of Sphaeroplea ; v, large non-motile egg, and minute 

 spermatozoids of Vaucheria. 



risks of youth. But the larger the egg the less mobile 

 it will be. Even in a fluid medium a large body is less 

 easily moved than a small one. We naturally associate 

 this with the fact that the larger eggs have lost their 

 motility. Motility of the egg is, however, immaterial so 

 long as the spermatozoids remain small and actively motile, 

 provided the egg can influence their movements so that it 

 shall act as a centre of attraction ; and this we have seen 



