MENDELISM AND SEX 137 



then one half of the sperm-cells or spermatozoa 

 will also contain five, but the other half will contain 

 four only. There are thus two kinds of spermatozoa 

 developed. We owe this interesting observation to 

 the investigations of Professor Wilson, Professor 

 Morgan, and Miss Stevens. Between them they have 

 examined about a hundred species of insects and 

 other animals, and in all of them they have found 

 this double form of spermatozoon present. 



Now it is very clear that if there are two kinds 



of spermatozoa in certain animals, the one 



kind approximately equal in number to the other, 



and there exists only one kind of egg-cell, in 



random fertilisations one half of the eggs will be 



fertilised by one kind of spermatozoa and the other 



half by the other kind. And from the fertilised 



eggs, fertilised by two different kinds of spermatozoa, 



it is to be expected that there will arise two kinds of 



individuals. Let us illustrate this by reference to 



the following diagram (Fig. 1). In this diagram 



A represents the spermatozoon carrying only four 



chromosomes, while B represents the other form of 



spermatozoon, that which carries five chromosomes. 



We may speak of the fifth or odd chromosome as 



the accessory one. The egg-cells are all of one type 



and carry five chromosomes ; they are represented 



in the diagram by C. Now when A and C unite it 



is clear that the zygote or fertilised cell D^ will 



contain only nine chromosomes, or one less than the 



zygote D^. This latter, which is formed when 



B and C unite, contains ten chromosomes. The 



