VIII LIMITA TIONS OF RECRESCENCE 399 



to every group of cells which is to form a part of the organism. 

 The question is in every case, To what degree the formative 

 possibilities of the cells in question have become unchangeable 

 and definitely restricted, to what degree they are still of a 

 general kind. That at the lower stages of development 

 these possibilities are still general we have already seen. We 

 know that in plants even of a high grade of organisation roots 

 may grow from the most various parts if these are brought 

 under suitable conditions. Indeed, according to Vochting's 

 experiments, any fragment of a willow may, as we have seen, 

 grow into a perfect tree,^ provided only that the fragment 

 contains a certain formative tissue, namely, cambium. These 

 facts might suggest the question whether a special cell-tissue 

 is not present also in the adult animal from which all parts 

 can be produced, a kind of animal cambium, a general forma- 

 tive tissue which retains general formative powers through- 

 out life. The blood will first occur to us as most likely to 

 fulfil these conditions, and especially the white blood-cells, 

 which take so prominent a part in constructive and destruc- 

 tive processes, — but we have no evidence that these can 

 produce any but mesodermic tissues, no evidence that 

 endodermic or ectodermic tissues can be produced in higher 

 animals from other than their own primitive layers. In fact, 

 it follows from the nature of heredity that they cannot. To 



1 I have already pointed out how impossible it is to reconcile such facts as 

 -this with the theory of the continuity of the germ-plasm. From the fragment of 

 willow is formed a willow tree with its reproductive organs, that is with germ- 

 plasm, and thus the latter is contained in the fragment— in Lunularia is contained 

 in any particle whatever. Van Bambeke {Poarquoi nous ressemhlons d no.s 

 parents ? Bruxelles, 1885) also draws attention to the same conclusion, pointing 

 out that from a piece of a Begonia leaf a whole plant with its flowers may pro- 

 ceed. But his assertion (p. 46), that Weismann assumes the presence of germ- 

 plasm in somatic-cells is evidently due to a misunderstanding. Weismann only 

 supposes that small quantities of germ-plasm are present in the somatic-cells of 

 hydroid polyps, " afterwards passing through an endless succession of cells till 

 they reach those remotest individuals of the colony in which the sexual products 

 are formed," etc. {Continuitdt, p. 61). But here the germ-plasm only excep- 

 tionally occurs in somatic-cells, and migrates from them to others. 



