80 



HISTOLOGY AND PHYSIOLOGY. 



and nucleus along with it. The nu- 

 cleus, the roundish or elongated body 

 usually present and imbedded in the 

 protoplasm, seems to be of the same 

 chemical composition as the latter, and 

 is, therefore, considered a somewhat dif- 

 ferentiated portion of it. Its function 

 is not well understood, though it seems 

 to take a prominent part in the initial 

 processes of cell-multiplication. Some- 

 times it contains a smaller body within 

 it, called the nucleolus. In all ordinary 

 tissue one nucleus is present in each cell ; 

 but in a few Algse each cell may contain several nuclei. 

 103. There are four typical methods of cell-formation. 

 In the method called rejuvenescence, the whole proto- 

 plasmic mass within the cell contracts, expels a portion of 

 its water, escapes through the broken cell-wall, and begins 

 an existence as a young cell (Fig. 185), Some of the 

 fresh-water Algse, as CEdogonium, illustrate this process. 

 In another common and beautiful fresh-water Alga (S2nro- 

 gyra) new cells arise by conjugation (Fig. 186). Two 

 cells lying near each other begin to protrude their walla 

 (Fig. 186, 1) until they 

 touch, then unite (Fig. 

 186, 2), and the partition, 

 between them becomes ab- 

 sorbed, allowing free com- 

 munication (Fig. 186, 3 

 and 4). In the meantime 



Fig. 184. Rotation of Protoplasm, the arrows indicating the direction : «, nucleus: 

 ch, chlorophyll grains. Fig. 185. Cell-formation, by rejuvenescence, \i\Qidogoniutn. 



