CHAPTER XXVI 



INTERNAL FACTORS OF SEX DETERMINATION 



MANY suggestions have been made concerning the internal 

 factors that determine sex. We may roughly classify these views' 

 under two headings ; namely, those in which the condition of the 

 parents when the germ-cells are liberated is supposed to deter- 

 mine the sex of the individual, 1 and second, those in which some 

 internal change that determines the sex is supposed to take 

 place in the germ-cells themselves irrespective of the condition 

 of the parents. The different hypotheses when brought to- 

 gether seem to include nearly all of the possible conditions that 

 might be imagined to determine the sex of the offspring. We 

 may group the different views under the following headings: 

 (i) the age of the parents; (2) the condition of germ-cells at the 

 time of fertilization ; (3) the vigor of the parents ; (4) the effects 

 of inbreeding; (5) the size of the egg; (6) the ratio of nucleus to 

 cytoplasm ; (7) the extrusion of the polar bodies ; (8) the forma- 

 tion of male and female spermatozoa and eggs ; (9) the influence 

 of fertilization; (10) the influence of the cytoplasm. 



Age of Parents 



The only data bearing on this question are those for man and 

 for some of the domesticated animals. Hofacker (1823) and 

 later Sadler (1830) brought together some statistics that seem 

 to show that when the father is older than the mother more 

 boys are born ; and when the mother is older than the father 



1 This influence, being in part external to the germ-cells themselves, might be 

 classified as an external influence in the same sense that food is an external factor. 

 There is no sharp line to be drawn in such cases between internal and external 

 factors. 



