40 THWAITES'S THEORY. [BOOK i. 



is the Lyngbya ferruginea, Agardh, a plant scarcely differ- 

 ing from Oscillatoria, except in the greater firmness of the 

 membranous sheath which invests each filament : the filaments 

 of this plant are composed of lenticular masses of endochrome 

 and during the early part of their growth are inclosed in a 

 membranous sheath ; from this, however, they emerge when 

 mature, and soon afterwards become broken up into the 

 separate masses of endochrome, each of which appears to be 

 held together by a kind of mucus, and not to be surrounded 

 by a cell membrane. 



" I now proceed to point out instances in which the cell-mem- 

 brane is seen to be of quite a secondary character ; and that 

 its development is regulated entirely by the condition of the 

 endochrome it contains, and that, in fact, it owes its existence 

 to this endochrome. The production of cell-membrane and 

 endochrome has the appearance frequently of being synchro- 

 nous, but the endochrome may sometimes be seen becoming 

 invested with a cell-membrane, and this may be well observed 

 during the formation of the spore of Zygnemaand other spe- 

 cies of Conjugate. Those who have paid attention to this 

 family of plants are well aware, that previously to the forma- 

 tion of the fruit, two cells unite by means of a short tube 

 developed from each, and through the canal formed by the 

 union of these the endochrome of one of the cells passes into 

 the other cell, becomes mixed with its endochrome, and sub- 

 sequently around this mixed endochrome a cell-membrane is 

 developed. This membrane would certainly appear to be 

 developed by the endochrome and not by one of the original 

 cell-walls, otherwise we could not expect it to be entirely 

 influenced as to its form and size by the contained endo- 

 chrome, but that there would be indications of its being inde- 

 pendent of this. The spore-membrane, however, not only 

 corresponds in extent with the contained endochrome, but if, 

 as is sometimes accidentally the case, the mass of endochrome 

 has become divided into two portions, each of these portions 

 becomes covered with a cell-membrane ; thus showing that 

 the relation is between these and not between either and the 

 original cell-wall. 



" An abnormal growth which sometimes takes place in the 



