306 T. H. Morgan 
another a + 4 to produce the female; since male sperm lacks both a 
and b the male has only one a + 6 derived from the egg. This 
interpretation is exactly the same as that for P. caryzcaulis, except, 
of course, the male sperm being rudimentary only one kind of 
individual results from the fertilized eggs. 
PART II 
CriTIcAL REVIEW OF THE RECENT LITERATURE 
INTRODUCTION 
Two contrasting views appear in an examination of the recent 
theories of sex determination, one qualitative, the other quantita- 
tive. On several former occasions (1903, 1905, 1906, 1907) I 
have urged the advantages of the latter interpretation as explana- 
tory of the facts. By a quantitative interpretation, however, [ 
do not mean that the female is simply male plus something else 
a view recently advanced by Castle, but that male and female are 
two alternative possibilities of the living material, which possibility 
is realized depending on quantitative factors. When these quanti- 
tative factors are internal there are produced in the germ cells those 
conditions that turn the scale one way or the other. ‘The gametes 
are not, therefore, male and female, but contain certain factors 
which, when combined, give rise, in an epigenetic fashion, to one or 
the other alternative. By a qualitative interpretation I understand 
that there may exist in the gametes certain bodies or substances 
containing the materials either for a male or for a female. These 
bodies or substances are usually thought of as separated, “‘segre- 
gated”’ in different gametes at some period, and are then recombined 
in fertilization in such ways as to insure two kinds of individuals. 
The qualitative and the quantitative hypotheses look at the 
problem of sex from different points of view. 
In order to bring out the contrast of these alternatives I have 
attempted to give in the following pages an analysis of the results 
from both points of view without attempting to disguise my pref- 
erence for the quantitative interpretation with important modifica- 
tions of that view. 
