GROWTH AND SENESCENCE. 



1017 



daughter-cells. As a consequence two kinds of spermatozoa are 

 formed in equal numbers, one kind possessing the z-chromosome, 

 and the other without this element. In the ova, on the contrary, 

 there is an even number of chromosomes, the ^-chromosomes are 

 paired like the others and the matured ova all have, therefore, an 

 z-chromosome. Much evidence has been brought forward to show 

 that when a spermatozoon containing an z-chromosome fertilizes 

 an ovum a female results, while an ovum fertilized by a sperma- 

 tozoon without an x-chromosome yields a male. This result is ex- 

 pressed in the following diagram in which the presence of an 

 x-chromosome is represented by an x. 



Matured spermatozoa. 



Matured ova. 



Fig. 307. 



The combinations 1, 3 and 1, 4 give rise to females, and the 

 combinations 2, 3 and 2, 4 give rise to females. It is said that 

 in some animals (aphids) only the spermatozoa containing an 

 a>chromosome mature, and as a result all the progeny from sexual 

 union are females. In some cases the male gamete has two 

 ^-chromosomes, but of unequal size, the larger one being then 

 designated as the z-chromosome and the smaller one as the 

 y-chromosomes. In the union with the ova the spermatozoa with 

 ^/-chromosomes produce males; the others, females. 



We owe the establishment of this important generalization 

 largely to American investigators (McClung, Wilson, Stevens). 

 It enables us to treat sex as a Mendelian characteristic trans- 

 mitted by certain hereditary units or determiners. When two 

 of these are united in a fertilized ovum a female results, while a 

 single one gives rise to a male. From this point of view it has 

 been possible to construct satisfactory schemata to explain the 

 many known instances of sex-linked inheritance, such as hemo- 

 philia and color-blindness in man. 



Growth and Senescence. The body increases rapidly after 

 birth in size and weight. It is the popular idea that the rate of 

 growth increases up to maturity and then declines as old age ad- 

 vances. As a matter of fact, careful examination of the facts shows 



