618 VETERINARY PHYSIOLOGY 



fact that only those ova which have the paternal X chromosome 

 will develop into females explains the transmission of certain 

 paternal characters through the daughters. In the human sub- 

 ject this is seen in colour blindness and in haemophilia, a disease 

 characterised by excessive bleeding from slight wounds. These 

 conditions are common in the male but very rare in the female. 

 But since the female gamete alone contains the paternal chromo- 

 some, the abnormal conditions can pass only through the females 

 to become manifest in the male progeny. 



Such observations suggest that the determination of sex 

 may be co-related with the Mendelian hypothesis. 



(3) In some animals, e.g. the frog, the nutritional condition of 

 the ovum may determine its sex. By varying the condition of 

 the eggs before fertilisation the proportion of males to females 

 may be modified. 



It would appear that in some animals the course 

 of development of the gamete is firmly established from 

 the first, that in others it may be determined by the X 

 chromosome in impregnation, while in a third group it may be 

 modified by the nutrition of the cell either before impregnation 

 or at the time of impregnation. How far this last factor acts 

 in the case of mammals we do not at present know. 



There is some evidence in invertebrates that the number of 

 X chromosomes in a cell may be reduced under certain con- 

 ditions, and thus a potential female cell changed into a 

 male cell. 



The problems of Heredity are fully dealt with in all text- 

 books of Biology, and therefore they need not be considered 

 here. 



II. The Gonads. 



{The structure of the organs of reioroduction must he studied 

 practically.) 



The development of the true gametic cells of the gonads 

 and of their ancillary cells has been already considered on 

 p. 605. 



While the individual is actively growing, the reproductive 

 organs are quiescent ; but, when puberty is reached, they begin 

 to perform their functions — the testes to produce spermatozoa. 



