IV 



THE SEX CHROMOSOMES 119 



An analogous process is found to take place in the male-producing 

 eggs. It will be remembered that there are three kinds of eggs : 



(i) The sexual eggs, which need fertilization and which all 

 develop into females. In them meiosis takes place in the usual 

 way, and the mature egg is left with 4 + Xg chromosomes. When 

 these are fertilized by spermatozoa, which, as we have just seen, 

 all have the chromosome formula of 4 + X2, all the resulting zygotes 

 are 8 +X2X0, i.e. females. 



(2) Parthenogenetically developing eggs which are going to 

 develop into females. These produce only one polar body without 

 reduction of chromosomes, and at the single maturation division all 

 the chromosomes divide as at a somatic mitosis ; the ripe egg (and 

 also the polar body) is therefore left with 8 + XoXg chromosomes. 



(3) Parthenogenetically developing eggs which are going to 

 develop into males. Again only one polar body is produced, but at 

 the single maturation division one X2 chromosome is left behind in 

 the anaphase and does not enter into the mature egg nucleus. This 

 is consequently left with ten chromosomes of composition S+X,. 

 This is shown in Fig. 54, C, D, where an Xg chromosome (plainly 

 double in Fig. 54, C) is seen left behind when the groups of chromo- 

 somes separate at anaphase. The dark body in the cytoplasm which 

 is being nipped off with the polar body in Fig. 54, D, is presumably 

 this double X chromosome. 



A very important point for the general theory of the sex chromo- 

 somes is the fact that the sex of the individual which is going to develop 

 from the parthenogenetic egg is in these cases determined before the 

 distribution of the sex chromosomes at polar body formation ; for the 

 male-producing and female-producing eggs are already differentiated 

 from one another by their relative sizes before this point is reached ; 

 the eggs which are going to eliminate an X, chromosome, and therefore 

 develop into males, are smaller than the female - producers. While 

 therefore we may probably still speak of the presence or absence of the 

 Xg chromosome as determining the sex of the individual, we must realize 

 that in this case its presence or absence is not a matter of chance, but 

 that there is some earlier factor which determines whether it shall be 

 eliminated at maturation or not, and which consequently is a sex-deter- 

 mining factor earlier in the chain of causation. 



In P. caryaecaulis this prior factor in sex determination must be sought 

 very far back, for not only are the male-producing and female-producing 

 eggs thus early differentiated from each other, but they are produced by 

 different females. Moreover, all the offspring of a single stem mother 

 are alike in respect to the type of eggs which they produce. Thus the 

 sex of the members of the sexual generation is determined by the con- 



