STRUCTURE.] THWAITES's THEORY. 41 



cells or long tubes of Vaucheria will serve well to illustrate 

 how immediately an increased production of cell-membrane 

 is consequent upon an additional development of endochrome.. 

 The cells of Vaucheria are occasionally found to be infested 

 with a species of Vorticella, an infusory animalcule. This 

 little animal is seen occupying large pear-shaped protu- 

 berances upon the frond of Vaucheria, in which it deposits 

 its ova. Now it is interesting to observe the mode in which 

 these peculiar protuberances are formed. The Vorticella 

 may, in some instances, be seen within the tube of the plant, 

 and from the slight alteration in the endochrome, it may be 

 inferred that the little animal has not been long present there; 

 in other cases it may be observed that the presence of the 

 Vorticella has caused an evident dilatation of the cylinder of 

 endochrome with a corresponding enlargement of the cell- 

 membrane ; whilst in other examples this dilatation has gone 

 on so as to have produced a large pear-shaped appendage to 

 the frond, within which the Vorticella may be seen moving. 

 But what I would wish particularly to draw attention to, is 

 the fact that the stimulus arising from the presence of the 

 Vorticella has been operating immediately upon the internal 

 surface of the cylinder of endochrome, causing an abnormal 

 development of this, accompanying and consequent upon 

 which has been a corresponding and regular development of 

 cell-membrane; showing that the amount of production of 

 cell-membrane is regulated by the growth of the endochrome." 

 The author then proceeds to remark " upon a structure 

 which is developed in greater or less amount in most Algae, 

 external to the cell-membrane, possessing some characters 

 in common with it, and probably in many cases performing a 

 similar office in the economy of the organism. The structure 

 I allude to is the mucus which surrounds the cells of Algae, 

 and in some species, such as in many of the Palmelleae, of 

 considerable extent, so as to make up by far the greater part 

 of the plant. In some of the Palmelleae indeed, the plant at 

 first sight appears to be composed of an amorphous gelatinous 

 mass, containing cells imbedded in it, and would lead to the 

 idea that this gelatinous mass is the matrix from which the 

 cells are developed, and to which they owe their origin ; but 



