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 ;;/, 

 groups of determinants for 

 the mesoderm structures, in 

 addition to their own proper 

 idioplasm, — 7n^ containing 

 those for the lateral muscles, 

 ni^ those for the ventral 

 blood-vessel, nfi those for the 

 'liver-cells' and mesodermic 

 part of the intestine, ;//^ those 

 for the segmental organs and the adjacent system of muscles, 

 vi^ those for the dorsal vessel, 7?i^ 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 



mf^l 



£nt 



I*.'-- 



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; ttt. Cells with accessory deter- 

 minants for the mesoderm; n. Cells 

 with accessory determinants for the 

 nerve-cord. 



