TROTOCOCCUS PLUVIALIS. 547 



Sometimes the enveloping cell is developed around each 

 secondary primordial cell, M^hilst still within the parent- 

 cell. 



After a certain number of generations of this kind, 

 their course is interrupted. A delicate double line is 

 perceptible around the primordial cell, which is the first 

 indication of the new, rigid, hgneous membrane, which 

 is formed around it (Fig. 46). The motile zoospore, 

 in other words, is again transformed into the still form. 

 The primordial cell is converted into a primordial sac, 

 and the cycle is closed. 



Besides these, which are the most usual modes of pro- 

 pagation, viz. that of the still cells into two, and of the 

 motile, into four secondary cells ; there are a number of 

 others which may be considered as irregular, and in 

 which forms are produced, which do not re-enter the 

 usual cycle until they have gone through a series of 

 generations. 



The cell-contents, for instance, of thp still form, instead 

 of dividing first into two, and then these again each into 

 two secondary cells, and so on, may at once be sub- 

 divided into four segments, as takes place in the motile 

 form. (Fig. 8.) 



More frequently and regularly, under certain circum- 

 stances, the cell-contents of the still form divide at once 

 into eight portions, which become naked zoospores of 

 small size (Figs. 31, 32). It is not quite clear what 

 becomes of this form of motile zoospore, but there 

 seems reason for believing that they occasionally develope 

 an enveloping cyst, and thus become encysted zoospores, 

 and occasionally secrete a cellulose tissue and become 

 still cells ; but most of them probably perish without any 

 further change. They would thus correspond with the 

 smaller motile spores observed by Thuret and A. Braun 

 in other Algae (Hydrodictyon, Achlya, the Fucoidese, &c.) 

 associated with the larger germinating spores, and which 

 themselves were deprived of the germinative faculty. 



In the same way that the " still" parent-cell may pro- 



