624 



A. S. PARKES 



different, causing more of one type than of the other to survive 

 the severe journey through the female organs to the ova 

 (as T. H. Morgan has suggested, ' Physical Basis of Heredity ', 

 1919). It is most probable that the disproportion which exists 

 between the sexes at conception in most mammals is con- 

 nected with this point. 



Text-fig. 3. 



its a-35 aw a« a-qs 



Frequency polygon of head lengths of spermatozoa of mouse. 



Summary. 



1. Chromosome dimorphism of the spermatozoa has been 

 shown for a variety of mammals, and in some cases this has 

 been shown to be correlated with dimorphism in the head 

 lengths of the spermatozoa. 



2. In the present paper this correlation has been extended to 

 the spermatozoa of man, the mouse, and the rat, in which 

 chromosome dimorphism of the spermatozoa had previously 

 been shown, and in which head length dimorphism seems to 

 exist. 



3. The interest of these results lies in the probability that 

 the histological difference in the X- and Y-spermatozoa may 

 account for the inequality of the sexes at conception in 

 mammals. 



