98 



thb: plant cell. 



while no analagous process takes place in animals, with the 

 exception, perhaps, of the Tunicata, which are able to manu- 

 facture cellulose for their outer casings. 



Reproduction in Sphcerella takes place in two ways, viz., 

 an asexual method and a sexual one, in the latter of which the 

 conjugation of two similar individuals takes place. In the 

 former, or asexual process, a single cell divides into two, 

 which become encapsuled by a gelatinous cyst, secreted by the 

 cell, and common to both individuals : further divisions arise, 

 resulting in the production of a number of cells, pairs of these 

 being enclosed in cysts, and the whole enclosed by a gelatinous 

 mass similar in structure to that enclosing the daughter cysts. In 

 this stage the whole mass is said to be in the palmella phase (see 



Fig. 75. — Two Cells of a Cilain of Diatoms (Mdosira). Note the 

 peripheral protoplasm, the central nucleus held in position by 

 "bridles," and the two somewhat irregularly shaped chlorophyll 

 bodies. The frustule of the Diatom is seen to be striated in a longi- 

 tudinal direction : the double nature of each frustule is not sho^vn in 

 the figure (see text). 



Fig. 74, 3), and, later on, the separate cells may be freed from 

 their cysts and become free-swimming organisms once more. At 

 certain times, however, reproduction takes place by the con- 

 jugation of two similar small ciliated motile cells which have 

 arisen by the division of the original cell into a large number of 

 equal ciliated individuals (see Fig. 74, 4 and 5). These 

 become freed by the bursting of the original cell-membrane of 

 parent-cell, and, whilst swimming freely, two of these bodies 

 approach one another, and meet by their anterior ciliated ex- 



