THE MECHANISM OF REPEODUCTION 45 



distribution absolutely parallels the distribution of sex. 

 There are two types. In the males of animals of Type A, 

 which includes numerous flies, beetles, grasshoppers and 

 bugs from among the insects, as well as representatives 

 from several orders of mammals, a single x-chromosome 

 is present in addition to the regular chromosome pairs, 

 and for this reason two kinds of spermatozoa are pro- 

 duced at spermatogenesis in equal numbers, those pos- 

 sessing the extra element and those without it. In the 

 females, on the other hand, two of these elements are 

 present and the eggs, therefore, always possess it. Thus, 

 on fertilization, half of the resulting young have two 

 x-chromosomes and these become females, while half have 

 but one and become males. 



Diagrammatically it is this : 



Ovum with x fertilized by sperm with x = female. 

 Ovum with x fertilized by sperm without x = male. 



In some other cases (Type B), the eggs are di- 

 morphic, while the sperm are all aUke, but the result 

 is the same ; the sex distribution follows the chromosome 

 differentiation. 



In dioecious plants there is some evidence of a similar 

 condition. Strasburger ^"^ found in one of the liverworts, 

 Sphserocarpus, where the four spores produced by a single 

 spore mother cell hang together and each such tetrad 

 could be planted separately, that invariably two males and 

 two females were produced. More recently Allen ^ has 

 presented evidence of an x-chromosome in this genus. His 

 discovery was made with material of the species SpJicero- 

 carpus Donnellii, but it has been corroborated by one of 

 his students working with SphcBrocarpus texanus. 



