PBOTOCOCCUS PLTJA'IAIJS. 545 



(Fig. 2). This form arises from the metamorphosis of the 

 encysted zoospore into the still. 



It may imdergo changes, which may be distinguished 

 as essential and non-essential. The latter have reference 

 to the change in form and colouring of the contents ; the 

 former, to propagation. 



The latter process takes place by the division or seg- 

 mentation of the contents, at first as above stated, into 

 two, then into four, eight, or sixteen, when the division 

 usually terminates, and the segments pass into the motile 

 form ; although further still generations may arise in the 

 same way. The segmentation takes place in the follow- 

 ing mode. Tn the first place the cell becomes elongated, 

 so that its diameter in one direction is twice as great 

 as in the other (Fig. 5). Then a constriction of the 

 contents is perceptible about the middle of the length of 

 the cell, which gradually deepens, and the cell contents 

 or primordial sac are divided into two halves (Fig. 6). 

 These are separated by a line which forms between 

 them ; and finally each is organised into a distinct 

 globule (Fig. 7), which becomes surrounded with a 

 ligneous membrane. The membrane of the parent-cell 

 is passive during this process — continuing, for a time, 

 to surround (sometimes in a gelatinous form) the secon- 

 dary cells ; it is finally dissipated, and they thus become 

 free. The part played by the supposed cytoblast in the 

 process of segmentation is very doubtful. 



During their dissolution, the cell-membrane of the de- 

 funct parent-cells is gradually converted into a mucoid 

 substance, retaining the secondary cells more or less in 

 connected masses. In this way arises the Prot. pluvialis, 

 leprosus, of Flotow. It is known also that other uni- 

 cellular AlgsB and Infusoria {Cldaniidomonas, Etiglena, 

 Monas, Vibrio, Zoospores, Diatomacese, &c.), under certain 

 circumstances, form similar envelopes, either by the 

 secretion of a mucoid substance, or the transformation of 

 their cell-membrane into such. 



After a certain number of such divisions, all in the 



35 



