EEPRODUCTION 617 



large and which manifestly undergoes division, multiplication, 

 and development to form the new individual, and one the sperm 

 or spermatozoon, which is generally smaller and which con- 

 jugates with the former. 



In vertebrates these are produced in separate individuals of 

 the species which have the distinctive anatomical and physio- 

 logical characters of the female and male. 



The dependence upon the gonads of the development of 

 these distinctive characters has been considered on p. 606 et seq. 



I. Determination of Sex. 



The problem of what determines in any ovum its develop- 

 ment either into ova associated with the female type of body or 

 into spermatozoa associated with the male type is a most difficult 

 one. A consideration of importance is that, under normal con- 

 ditions of sexual reproduction, the number of males and of 

 females produced is approximately equal. 



(1) In some animals the sex is determined before the ovum is 

 impregnated. Among vertebrates this seems to be the case in 

 birds. A hen with barred pattern of feathers transmits this 

 character to male chicks only, unless the father is also barred. 

 Hence it seems clear that there must be two kinds of ova — one 

 to produce males and one to produce females. 



(2) In other animals, and almost certainly in mammals, the 

 sex seems probably to be determined by there being two types 

 of spermatozoa. It is probable that certain special chromosomes, 

 often called the X chromosomes, are the determining factors. 

 When two of these are present, a female develops, when one is 

 present, a male ; and the gametic cells developed in each sex will 

 be characterised by the possession of these numbers. When these 

 cells undergo their reduction division (pp. 619 and 620), before 

 forming ova and spermatozoa, each female cell is left 

 with one X chromosome, while half the male cells contain 

 an X chromosome and half do not contain one. When one of 

 the former impregnates an ovum, that ovum has two X chromo- 

 somes and develops into a female; when one of the latter per- 

 forms the impregnation, the ovum has only one X chromosome 

 and develops into a male. 



The existence of these two kinds of spermatozoa and the 



