584 



PHYSIOLOGY. 



CHAPTER III. 



GROWTH AND DEVELOPMENT OF ORGANS. 

 Sect. 1. DEVELOPMENT or CELLS. 



The formation of new cells takes place in several ways,- though 

 essentially the process always consists in the setting apart of the 

 whole, or more frequently of a portion or portions of the proto- 

 plasm in one or other of the following manners : 1, by condensa- 

 tion or alteration of the molecular structure; 2, by segmentation or 

 by both combined ; and, 3, by the blending of the contents of one 

 cell with those of another. In any case the mass of protoplasm 

 newly set apart is sooner or later invested by a cellulose coat. It is 

 hence convenient to treat of cell-formation under the heads of 

 segregation, conjugation, segmentation, and free-cell formation. 



Cell-formation by Segregation. A certain portion (or portions) 

 of the protoplasm is set apart from the rest, escapes ultimately 

 from the cell-wall, lives independently for a time, and ultimately 

 is invested with a cellulose coat. The zoospores of (Edoyonium 



Development of zoospore in (Edogonium: a, parent filament; b, a joint breaking across to 

 emit its contents; c, a more advanced stage, the globular mass of contents (nascent zoo- 

 spore) still within a cellulose pellicle; d, empty parent cell; e, the zoospore escaped 

 from it, with its crown of cilia formed ; f, the zoospore, after it has settled down, become 

 encysted by a cellulose coat, and begun to grow into a new filament. Magn. 200 diam. 



and other filamentous Algae are formed in this manner (fig. 591). 

 Sometimes the whole of the protoplasm escapes from the cavity of 

 the cell, and ultimately forms a new cell. This is spoken of as re- 

 juvenescence. The detached masses of protoplasm often move in an 



