ORGANIC DIFFERENTIATION 83 



primordial conditions of generation — in other words because he 

 reversed the entire order of facts — that Weismann believed 

 himself justified in considering the plasma constituting the 

 generative organs — the germen or germ-plasm — as distinct 

 from the plasma constituting the organs of the body proper, 

 the somatic plasm or soma. Upon this distinction he built up 

 his whole system, a veritable pyramid resting on its apex — 

 but we need not pursue the matter further. 



The elements thus separated may evolve with varying 

 rapidity, and produce others more or less specialized, such as 

 those of the genital ducts and others that retain their primitive 

 character of blastomeres. The latter are naturally capable of 

 developing without impregnation into new individuals. This 

 explains most cases of parthenogenesis occurring in minute 

 insects such as plant-lice, the cochineal insects and Cynips, 

 occasionally complicated by viviparity, and liable to occur even 

 in larvae, such as those of the Diptera of the genus Cecidomyia. 



At the same time these reserve elements are not exclusively 

 destined to produce the genital apparatus ; they may belong 

 to any part of the embryonic body, and thus as in embryos 

 produced from predetermined blastomeres, represent that 

 particular portion of the body and none other. Thus Kiinckel 

 d'Herculais and Weismann have shown that in each of the 

 segments of insect larvae subject to complete metamorphosis 

 there are gathered together in the form of folds of the skin cer- 

 tain neutral elements to which the first of these naturalists gave 

 the name of Jiistoblasts, and the second imaginal discs. 

 They are destined either to replace outworn elements of the 

 larvae, or to produce those new tissues or organs which give the 

 adult animal a form often very different from that of the larva, 

 thus constituting an actual metamorphosis. The fact that these 

 histoblasts represent only that particular segment of the body 

 with which they are connected, tends, moreover, to establish 

 the individuality of these segments. This idea has its 

 importance in the explanation of organic evolution. 



We may postulate that the ultimate cause of this evolution 

 lies in the power acquired by the earliest microscopic organisms 

 to unite and to form a body destined to acquire tremendous 

 possibilities ; if further we may conceive that each element 

 in such an association retains a considerable degree of 

 independence, yet modifies itself in a special way according to 



