254 RECENT CYTOLOGY 



chromosome in its somatic cells than the male, thus 

 making up an even number that is to say, in the 

 female the pair to the odd chromosome of the male is 

 present, so that there are two heterotropic chromo- 

 somes, or idiochromosomes. These fuse and separate 

 in the reducing division, which thus proceeds in the 

 normal manner in this sex. In the male, on the other 

 hand, when the reducing division occurs, the hetero- 

 tropic chromosome passes complete into one of the 

 resulting cells. In the second gamete -producing divi- 

 sion, every chromosome present having divided into 

 two, the products oi this division pass into different 

 gametes. These latter divisions are of two kinds, 

 since in one of them the heterotropic chromosome takes 

 part, whilst in the other it is wanting ; consequently, 

 two out of the four spermatozoa eventually produced 

 contain the heterotropic chromosome and two do not. 

 (Only one of each kind is shown in Fig. 47.) Thus 

 there is a differentiation of the spermatozoa into two 

 different kinds, and one of these kinds contains a 

 chromosome less than the other. On the other hand, 

 all the eggs (as well as the polar bodies) contain an 

 idiochromosome . 



In fertilization some of the eggs become impregnated 

 by spermatozoa containing the heterotropic chromo- 

 some. Such eggs invariably develop into females 

 having a pair of idiochromosomes in each somatic 

 cell. Other eggs are fertilized by spermatozoa lacking 

 the heterotropic chromosome, and these become males, 

 their somatic cells containing only the single hetero- 

 tropic chromosome derived from the egg. The ac- 



