xn] NATURE OF DETERMINANTS 187 



istic of the body-cells, and yet the cause of the difference 

 seems rather to lie in the pole-plasma than in the additional 

 chromatin. In Ascaris^ although no visible cytoplasmic 

 factor is present, the experimental evidence points to a 

 similar conclusion, and it may therefore be important to 

 keep in mind the possibility that a body may be a constant 

 feature of the germ-cells as distinguished from body-cells, 

 and yet not be the cause of the difference in function and 

 behaviour. 



GATENBY'S suggestion in his work on Tricho gramma 

 (1918 b) that "the function of the determinant is nutrimen- 

 tal" may well be true, of similar bodies in other forms. If so, 

 it may be supposed that the various forms of germ-cell deter- 

 minants are at least usually either themselves masses of re- 

 serve material, or are the visible expression of the existence 

 of supplies of such material in the cells, which result in delay- 

 ing or preventing cell differentiation, and so cause the cells 

 x to remain in an embryonic condition. The body-cells would 

 thus undergo differentiation, while the cells containing the 

 "germ-cell determinant" would remain in their primitive 

 unmodified condition, and so be capable at a later stage of 

 giving rise to germ-cells 1 . 



1 GATENBY (1918^) concludes that in the polyembryonic Hymenoptera 

 (Litomastix, etc.) there is no evidence of a " germ-track," and that SILVESTRI'S 

 oosoma " has no other effect than that of temporarily stopping mitosis in the 

 cells which happen to contain it." But in these species the egg divides at the 

 beginning of development into an anterior part which forms the embryonic 

 envelope, and a posterior part which forms the cells that give rise to the 

 numerous embryos (Fig. 20; cf. also Fig. 10, p. 82). This suggests the 

 speculation that the posterior part may perhaps be regarded as a single primitive 

 ovum, which divides into many ova, and that these then undergo partheno- 

 genetic development into larvae. If this were true, the polyembryony of such 

 forms as Litomastix could be correlated with' the alternation of sexual and 

 parthenogenetic generations in the Cynipidae. 



