THE CELL-THEORY 267 



character of the changes undergone — of the different states necessarily 

 exhibited — or, in other words, the successive differentiations of the 

 amorphous mass will be different. 



Conceived as a whole, from their commencement to their termination, 

 they constitute the individuality of the living being, and the passage 

 of the living being through these states is called its development. 

 Development, therefore, and life are, strictly speaking, one thing, 

 though we are accustomed to limit the former to the progressive half of 

 life merely, and to speak of the retrogressive half as decay, consider- 

 ing an imaginary resting point between the two as the adult or 

 perfect state.^ 



The individuality of a living thing, then, or a single life, is a 

 continuous development, and development is the continual differentia- 

 tion, the constant cyclical change of that which was, at first, morpho- 

 logically and chemically indifferent and homogeneous. 



The morphological differentiation may be of two kinds. In the 

 lowest animals and plants— the so-called unicellular organisms — ^it 

 may be said to be external, the changes of form being essentially 

 confined to the outward shape of the germ, and being unaccompanied 

 by the development of any internal structure. 



But in all other animals and plants, an internal morphological 

 differentiation precedes or accompanies the external, and the homo- 

 geneous germ becomes separated into a certain central portion, which 

 we have called the endoplast, and a peripheral portion, the periplast. 

 Inasmuch as the separate existence of the former necessarily implies 

 a cavity, in which it lies, the germ in this state constitutes a vesicle 

 with a central particle, or a " nucleated cell." 



There is no evidence whatever that the molecular forces of the 

 living matter (the " vis essentialis " of Wolff, or the vital forces of the 

 moderns) are by this act of differentiation localized in the endoplast, 

 to the exclusion of the periplast, or vice versa. Neither is there any 

 evidence that any attraction or other influence is exercised by the one 

 over the other ; the changes which each subsequently undergoes, though 

 they are in harmony, having no causal connexion with one another, 

 but each proceeding, as it would seem, in accordance with the general 

 determining laws of the organism. On the other hand, the " vis 

 essentialis " appears to have essentially different and independent ends 

 in view — if we may for the nonce speak metaphorically — in thus 

 separating the endoplast from the periplast. 



^ Dr. Lyons, in his interesting ' Researches on the Primary Stages of Histogenesis and 

 Histolysis,' has invented a most convenient and appropriate term for this latter half of 

 development, so far as the tissues are concerned — viz., Histolysis. 



