50 THE CELL DOCTRINE. 



the periplast. Inasmuch as the separate existence of 

 the former necessarily implies a cavity in which it 

 lies, the gerrn 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 liv- 

 ing matter (the 'vis essentialis' of Wolf, or the 

 vital forces of the moderns), are by this act of differ- 

 entiation 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 sub- 

 sequently undergoes, though they are in harmony, 

 having no causal connection 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 es- 

 sentially different and independent ends in view, in 

 thus separating the endoplast from the periplast. 



"The endoplast grows and divides; but, except in 

 a few more or less doubtful cases, it would seem to 

 undergo no other morphological change. It fre- 

 quently disappears altogether; but as a rule it under- 

 goes neither chemical nor morphological metamor- 

 phosis. So far from being the centre of activity of 

 the vital actions, it would appear much rather to be 

 the less important histological element. 



" The periplast, on the other hand, which has hith- 

 erto passed under the name of cell wall, contents 

 and intercellular substance, is the subject of all the 

 most important metamorphic processes, whether 

 morphological or chemical, in the animal and plant. 

 By its differentiation, every variety of tissue is pro- 



