1000 THE PHYSIOLOGY OF REPRODUCTION. 



100 female, and the data from other countries show the same 

 fact of an excess of male children. Owing to the greater death-rate 

 of the male, the proportion of male to female in the adult population 

 of Europe is as 1000 to 1024. Examination of these statistics with 

 reference to determining conditions led to the formulation of the so- 

 called Hof acker-Sadler law or laws, which may be stated as follows: 

 (1) When the man is older than the woman the ratio of male 

 births is increased (113 to 100). (2) When the parents are 

 of equal age the ratio of female births is increased (93.5 males to 

 100 females). (3) When the woman is older the ratio of female 

 births is still further increased (88.2 to 100). These laws have 

 been corroborated by some statisticians and contradicted or modi- 

 fied by others. Ploss attempted to show that poor nutritive con- 

 ditions affecting the parents, especially the mother, favor the 

 birth of boys. Dusing combined these results in a sort of general 

 compensatory law of nature, according to which a deficiency in 

 either sex leads, by a process of natural selection, to an increase 

 in the births of the opposite sex. Thus, when males are few in 

 number, as the result, for instance, of wars, females marry 

 later and more males are produced. When males are in excess early 

 marriages are the rule and this condition favors an excess of female 

 births. However interesting these statistics may be, it is very 

 evident that they do not touch the real problem of the cause of the 

 determination of sex. 



Modern work has turned largely to observations and direct 

 experiments upon the lower animals, and particularly to cytolog- 

 ical studies of the reproductive cells. The trend of this work tends 

 to oppose an older view founded largely upon experiments on frogs, 

 bees, and wasps, according to which the sex is controlled or may be 

 controlled by the conditions of nourishment during development, 

 favorable conditions of nutriment leading to the development of 

 female cells from the germinal epithelium of the embryo. On 

 the contrary, it is now believed that while such external conditions 

 may affect the sex ratio, the factors that actually determine sex are 

 internal and the sex of the fertilized ovum is fixed at the time of 

 fertilization. The view that holds at present connects the deter- 

 mination of sex with the presence or absence of an accessory 

 chromosome, the z-chromosome, in the matured spermatozoon. 

 In many animals the spermatocyte, like the other cells of the male, 

 contains an odd number of chromosomes. In man, for example, 

 the number is given as 47. When synapsis of these chromosomes 

 occurs in the process of maturation, 23 pairs are formed and one 

 odd one, the z-chromosome. At the reduction division to form the 

 spermatozoa these pairs separate, one going to each daughter-cell, 

 but the z-chromosome being unpaired goes to one only of the 



