56 THE CELL DOCTRINE. 



kinds. In the lowest animals and plants, — the so- 

 called unicellular organisms — it may be said to be ex- 

 ternal, the changes of form being essentially confined 

 to the outward shape of the germ, and being unac- 

 companied by the development of any internal struc- 

 ture. 



" But in all other animals and plants, an internal 

 morphological differentiation precedes or accompa- 

 nies the external, and the homogeneous 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 liv- 

 ing matter (the ' vis essentialis ' of VV'^olff, or the vital 

 forces of the moderns), are by this act of differentia- 

 tion localized in the endoplast to the exclusion of 

 the periplast, or vice versa. Neither is there any evi- 

 dence that any attraction or other influence is exercised 

 hy the one over the other ; the changes which each sub- 

 sequently ondergoos, 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 (nucleus) grows and divides ; but, 

 except in a few more or less doubtful cases, it would 

 seem to undergo no other morphological change. It 



