THE CELL 



53 



its of the cell, changing neither its shape nor position. It occurs in 

 both plant and animal cells, and consists of a sort of circulation or 

 "streaming " of the protoplasm. It is evidenced by the movement of 

 minute granules present in the protoplasm, by changes in the position 

 of the nucleus, etc. 



(c) Ciliary Movement. This is the whipping motion possessed 

 by little hair-like processes called cilia, which project from the sur- 

 faces of some cells. 



Certain cells which are specialized for the particular purpose of 

 motion as, e.g., muscle cells, possess such powers of contraction that 

 they are able to move not only themselves but other parts with which 

 they are connected. This 

 power of contractility is de- 

 pendent upon the spongio- 

 plasm, the hyaloplasm play- 

 ing a more passive role. In 

 muscle cells the highly de- 

 veloped contractile powers 

 appear to be due to the ex- 

 cessive development and 

 peculiar arrangement of the 

 spongioplasm. 



5 . Reproduction. — The 

 overthrow of the long-held 

 biological fallacy of sponta- 

 neous generation was soon 

 followed by the downfall of a similar theory regarding the origin 

 of cells. We now know that all cells are derived from cells, and 

 that the vast number and complex of cells which together form 

 the adult human body are all derived from a single primitive cell, 

 the ovum. 



Reproduction of cells takes place in two ways, by direct cell 

 division or amitosis, and by indirect cell division or mitosis. In both 

 amitosis and mitosis the division of the cell body is preceded by divi- 

 sion of the nucleus. 



Direct Cell-dwision — Amitosis (Figs. 7 and 8). — In this form of 

 cell-division the nucleus divides into two daughter nuclei without any 

 apparent preliminary changes in its structure. The division of the 

 nucleus may or may not be followed by division of the cell body, in the 

 latter case resulting in the formation of polynuclear cells. This form 



Fig. 7. — Epithelial Cells from Ovary of 

 Cockroach, Showing Nuclei Dividing Araitot- 

 ically. (Wheeler.) 



