THE HEREDITY OF SEX 41 



and heredity was the transmission of constitutional 

 characters from parent to offspring. Now that 

 we fix our attention on the fertilised ovum or the 

 gametes by which it is formed we see that the 

 characters are determined by some properties in 

 the constitution of the gametes. What, then, is 

 heredity ? Clearly, it is merely the development 

 in the offspring of the same characters which were 

 present in the ova from which the parents developed. 

 When the characters persist unchanged from genera- 

 tion to generation, we call the process by which they 

 are continued heredity. When new characters 

 appear, i.e. new characters determined in the ovum 

 not due to changes in the environment, we call them 

 variations. When a fertilised ovum develops into 

 a new individual, it divides repeatedly to form a 

 very large number of cells united into a single mass. 

 Gradually the parts of this mass are differentiated 

 to form the tissues and organs of the body or soma, 

 but some of the cells remain in their original con- 

 dition and become the reproductive cells which will 

 give rise to the next generation. The reproductive 

 cells also undergo division and increase in number, 

 and when they separate from the new individual 

 and unite in fertilisation they still possess all the 

 determinants of the fertilised ovum from which 

 they are descended. Heredity thus continues from 

 gamete to gamete, not from zygote to soma, and 

 then from soma to gamete. 



Modern researches have shown that the nucleus, 

 when the cell divides, assumes the form of a spindle 

 of fibres, associated with which are distinct bodies 

 called chromosomes, that the number of these 

 chromosomes where it can be counted is constant 



