530 THE NATURAL HISTORY OF 



Microscopical analysis, therefore, demonstrates, in the 

 still cells of Protococcus, whatever aspect they may pre- 

 sent, the following elements : 



1. A closed cell-membrane; 2. contractile, sometimes 

 colourless, sometimes green, sometimes red, cell-contents : 

 the latter in innumerable droplets, the two former fre- 

 quently condensed or separated into more solid granules ; 

 3. lastly, one or more chlorophyll vesicles, and in certain 

 stages a cytoblast. All these elementary parts occur also 

 in the cells of other plants, and there is, consequently, no 

 difficulty in the referring of all such forms, when in the 

 state of rest (the special value of which is only shown by 

 the history of their after-development) to the simple 

 vegetable cell. 



This inquiry, however, is far more difficult in the 

 case of the much more variously constructed motile 

 form. 



This form was considered by Flotow to consist of a 

 larger parent vesicle, surrounding numerous smaller red 

 and green cellules, and itself again surrounded by a 

 mucous envelope, within which were two tentacula, often 

 placed upon a beak-like projection. 



The first thing ascertained by Cohn was the non- 

 existence of the mucous envelope. The optical appear- 

 ances distinctly showed the existence of, not an enveloping, 

 at all events fluid mucus, but of a solid membrane. 



Although this statement does not accord with the 

 notions hitherto entertained with respect to similar 

 appearances of an enveloping mucous layer in other 

 Algae — which has been regarded by Kiltzing as a gela- 

 tinous or mucous envelope, consisting of jelly (gelin), and 

 termed by him a gelatinous cell (gelin-zelle) or tube, 

 or sometimes amorphous gelatine ; and which has been 

 designated by Nageh as an "enveloping membrane (hiill- 

 membran), who supposes its outer layer to consist of a 

 homogeneous, semifluid gelatinous substance, — Cohn 

 states, that at all events, in the case of Protococcus, a struc- 

 ture so abnormal and different to what exists in all other 



