SECT. ii. VOL VOCINEJE. 1 87 



x 



the transparent space is only occupied by a watery 

 liquid. The varieties of this plant are very numerous, 

 and all related to one another. Sometimes the whole 

 of the matter within the primordial cell of the spore 

 divides at once into 4, 8, or 16 parts, giving rise to as 

 many minute primordial cells. 



The cilia are extensions of the colourless transparent 

 film which covers the zoospores, and their vibrations 

 are generally believed to be a consequence of the vital 

 contractibility of that film, and intimately related to 

 the changes taking place in the cell on which they are 

 borne. The persistence of their motions after a cell 

 is detached from a compound body covered with them, 

 being like the persistence of the contractibility of 

 muscle fibre after being detached from a living animal, 

 proves that we must look to a contractile energy in the 

 film of protoplasm for the maintenance of these curious 

 operations. 



It appears that a cell cannot perform two functions 

 at the same time, and that one must either precede or 

 follow the other. Thus, the zoospores have two dis- 

 tinct periods of action ; the first is that of mechanical 

 motion alone, which is followed by one of growth and 

 multiplication, manifestations which, though very dis- 

 similar, are really modes of action of the same vital 

 energy that formed these bodies while they were yet in 

 their parent cell. In fact, it seems to be a general law, 

 that each cell is endowed, altogether or for a time, with 

 its own mode of action, and is incapable of any other. 



Of the Volvocinese, by some regarded as fresh-water 

 microscopic plants, the Stephanosphsera pluvialis may be 

 taken as a type. This plant consists of a colourless trans- 

 parent globe not more than the -^g^th of an inch in 

 diameter, containing eight green primordial cells ar- 

 ranged in a circle in its equator. Each primordial cell is 

 furnished with a pair of cilia; these 16 cilia pierce through 



