902 THE PHYSIOLOGY OF REPRODUCTION. 



fertilization before menstruation favoring male births, after men- 

 struation female births; that it depends upon the nutritive con- 

 ditions of the ovum during development or of the maternal parent ; 

 that it depends upon the relative ages of the parents; that there 

 are preformed male and female ova and male and female sper- 

 matozoa, etc. What we may call the scientific study of the problem 

 began with the collection of statistics of births. Statistics in Europe 

 of 5,935,000 births indicate that 106 male children are born to 

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

 fact of an excess of male children. 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, particularly the inverte- 

 brates, with the result that a very large number of facts have 

 been collected of a most interesting kind, but difficult as yet to 

 interpret so as to formulate a general law. The trend of modern 

 work tends to oppose an older view founded largely upon experi- 

 ments on frogs, bees, and wasps, according to which the sex is not 

 determined at or before fertilization, but 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. In 

 contrast with this latter view an opinion that has been frequently 

 advocated is that the sex of the embryo is determined in the egg 

 before fertilization or at the time of fertilization. This view assumes 

 substantially that there are male and female eggs to begin with, and 



