EVOLUTION 359 



means that the germ-plasm which contains the factors 

 determining the inherited constitution of offspring and 

 descendants is somehow affected by the experiences and 

 actions of the creature in whose body the germ-cells are 

 developed. Scepticism as to this influence has been in- 

 creasingly dominant in the minds of those who have studied 

 the subject since the promulgation by A. Weismann (1893) 

 of his '' germ-plasm " theory, according to which there is 

 a cardinal distinction between the essential substance of the 

 germ-nuclei and the nuclei of the various tissue-cells of 

 the body (" somatic " cells), and especially since the demon- 

 stration of the great importance of the germ-nuclei in 

 inheritance which has come through recent cytological and 

 Mendelian research (see pp. 118-131). A detailed exposi- 

 tion and discussion of Weismann's theory would be out of 

 place here, but his essential contention is that the germ- 

 cells in each generation must be regarded as originating 

 from the germ-cells of the preceding generation, and not 

 from the body-cells of the creatures that carry them ; 

 hence the germ-plasm is continuous through all the genera- 

 tions of a race, while the bodies are evanescent. This view, 

 though to a great extent theoretical, derives observational 

 support from those cases in which the germ-cells of a 

 developing animal are recognisable at a very early stage as 

 distinct from the body-cells. The early observations of 

 Weismann (1863) on the embryology of chironomid midges 

 and of E. Metschnikoff (1866) on the abnormally developing 

 virgin eggs of cecidomyid larvae, demonstrated, at the 

 hinder pole of the segmenting egg, cells of distinct appear- 

 ance ; Metschnikoff and later observers traced the ova or 

 sperms from these '* pole-cells " (Fig. 80, g) through what 

 is now known as a definite " germ- track." Such germinal 

 pole-cells have now been detected in the embryos of insects 

 of several orders besides the Diptera — Orthoptera, Coleo- 

 ptera, Hymenoptera, for example ; R. W. Hegner (19 14), 

 one of the recent students in this line of inquiry, has given 

 a good general account of the distinctive granular or other 

 inclusions in the early differentiated germ-cells, and it is 



