148 



THE GERM-PLASM 



tained in them becomes disintegrated in the course of the sub- 

 sequent divisions, is strictly definite, and determines the number 

 of successors which each cell produces, as well as the relative 

 position and combination into organs, and histological differen- 

 tiation of the cells. When the process of proliferation begins, 

 the newly-formed cells no longer retain the specific epidermic 

 character, and their successors may, indeed, be said to possess 

 an 'embryonic character,' in the sense in which that term has 

 usually been used, — if it is not thereby understood that they 

 must all contain similar primary constituents. Their further 

 development shows that this cannot be so : the cells of one 

 particular region give rise, for instance, to the dorsal vessel ; 

 those of another to the nerve-cord ; those of a third region to 

 certain muscles, and so on. 



We must therefore suppose that the various epidermic cells 



of the parent animal are pro- 

 ^ vided with active accessory 



idioplasm, somewhat in the 

 manner I have indicated in 

 the accompanying diagram. 

 The cells marked «, for in- 

 stance, would contain the 

 groups of inactive determi- 

 nants for the formation of the 

 ventral nerve-cord ; those of 

 the epidermis marked m^ 

 groups of determinants for 

 the mesoderm structures, in 

 addition to their own proper 

 idioplasm, — )n^ containing 

 those for the lateral muscles, 

 in- those for the ventral 

 blood-vessel, nt^ those for the 

 'liver-cells' and mesodermic 

 part of the intestine, ;//■* those 

 for the segmental organs and the adjacent system of muscles, 

 ifv" those for the dorsal vessel, in^ those for the dorsal muscles, 

 and so on. For the sake of simplicity I have supposed that the 

 epidermis consists of only one layer of cells, although in reality 

 there are two layers in many parts : the diagram, in fact, is not 

 by any means intended to represent the actual structure of the 



IFtrtT-' 



Enc 



*n»-'. J 



Fig. 5. — Transverse section through a Nais 

 in the region of the zone of regeneration 

 (modified from Semper) : Ekt, Integu- 

 ment; Ent, Epithelium of the ali- 

 mentary canal; N, Nerve-cord; Ms, 

 Visceral mesoderm; Vd, Dorsal blood- 

 vessel; in. Cells with accessory deter- 

 minants for the mesoderm; n. Cells 

 with accessory determinants for the 

 nerve-cord. 



