CHAPTER XXVI 
INTERNAL FACTORS OF SEX DETERMINATION 
MANvy 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,’ 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: 
(1) 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. 
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