546 THE NATURAL HISTORY OF 



still form — usually after the third or fourth — the ultimate 

 segments, instead of surrounding themselves with a lig- 

 neous membrane, become free, in the naked condition, 

 and developing the two motile filaments, represent a 

 motile primordial cell. (Pig. 12.) 



The way in which the vibratile cilia are produced is 

 quite obscure. 



The production of motile zoospores may take place on 

 the first segmentation of the contents of the primary, still 

 form, or after the intervention of an uncertain number of 

 such divisions as said before ; but it is clear that the 

 segments of the contents of a still cell may at any time as- 

 sume the motile condition, and that their prolonged reten- 

 tion of the still form depends upon various external con- 

 ditions. But with division, on the other hand, it does not 

 appear possible for the still form to be changed into the 

 motile, whilst the contrary may undoubtedly take place. 



The production of the enveloping cell around the 

 primordial cell is then described, as in a former part of 

 the paper; and the analogy between this process and 

 that of cell- formation, given by Mohl, Nageli, Schleiden, 

 &c., pointed out. (Figs. 16, 17, 20, 21.) 



The encysted zoospores, thus constituted, grow for a 

 time very considerably, and after a certain time exhibit 

 a tendency to propagate, that is to say to divide. 



The contents of the primordial cell (which alone is 

 potential in this act) exhibit lines of division into four 

 symmetrical portions, most distinctly shown by the chlo- 

 rophyll vesicles. Then, in directions corresponding to 

 these lines, the cell-contents are divided into four con- 

 tiguous portions, which gradually become isolated, and 

 assume a globular form, afterwards develope the two 

 vibratile ciha, and upon rupture of the parent-cell, be- 

 come free, and swim away. (Figs. 12, 13.) 



A new enveloping cell is afterwards developed, and 

 these secondary motile cells become a second generation 

 of encysted spores, which, though in some respects unlike, 

 yet must be regarded as equivalent to their parent. 



