ONTOGENESIS 241 



I. Theories diminishing in validity with increasing 



knowledge of cytological science. 

 1. That sex determination depends upon conditions 



entirely apart from the germinal cells. 

 A. That the sex of the offspring is determined by 

 the condition of the parents: 



(a) By the age of the parents. Sadler taught 

 that the sex of the offspring was in large meas- 

 ure determined by the age of the father. He 

 collected statistics that seemed to show that 

 the older the male parent was the greater the 

 number of male offspring he produced. 



(6) By the vigor of the parents. Popular belief, 

 said to be based upon results obtained in breed- 

 ing domestic animals, has led to the opinion 

 that the sex of the offspring corresponds to that 

 of the less vigorous parent. Almost as many 

 facts, however, favor the opposite opinion, that 

 it is the more vigorous parent that determines 

 the sex of the offspring. 



(c) By the nutrition of the parents. The whilom 

 popular theory of Schenk taught that the 

 nutrition of the mother was at the foundation 

 of sex determination. If, during pregnancy, 

 she was kept in the highest state of nutrition, 

 female offspring were more numerous; if, on 

 the other hand, her nutrition was kept below 

 par, there was greater likelihood of male off- 

 spring. 



In all three cases it would seem as though the 

 circumstance of the occurrence of both sexes 

 in cases of plural births would overthrow the 

 validity of the theory. Thus twins are fre- 

 quently of both sexes, and, among animals 

 simultaneously giving birth to many offspring 

 at the same time, the sexes are usually almost 

 equally divided. Moreover, these theories 

 could in no way be made to apply in the cases 



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