CHAPTER XII 



Germ-cell Determinants 



/> II ^HE evidence given in the preceding chapter points 

 strongly to the conclusion that in many animals sex 

 is determined by, or at least is correlated with, the presence 

 or absence of a particular chromosome in the fertilised egg, 

 or, among parthenogenetic animals, in the egg after matur- 

 ation. Before proceeding to consider whether other chromo- 

 somes may be regarded as determining the presence or 

 absence of other inherited characters in the same sort of 

 way, it will be convenient to make a short digression in 

 order to describe how the separation of the germ-cells from 

 the other cells of the organism may be dependent on the 

 presence of definite bodies which are often known as "germ- 

 cell determinants." 



The conception of the "continuity of the germ-cycle" 

 was introduced by WEISMANN, and was based largely on his 

 researches into the origin of the germ-cells of Diptera and 

 Hydrozoa. The essence of the idea, at least in its original 

 form, was that the germ-cells are not produced by the body, 

 but exist as an independent cycle, continuous from genera- 

 tion to generation, living in and nourished by the body, but 

 not formed from it. According to this conception, when an 

 egg segments, some cells are from the first marked off from 

 the rest as primitive germ-cells; they remain undifferenti- 

 ated while the other embryonic cells become modified into 

 the various tissues, and when the organism is adult they 

 give rise to ova or spermatozoa. The fertilised egg thus 



