4 THE CELL AND SIMPLE ANIMAL TISSUES. 



This living matter may also be observed in a naked or free 

 state (AniosbcB). 



This all - important living substance is endowed with the 

 powers of contractility and movement, and is subject to such 

 external influences as light, heat, and electricity. Movement 

 takes place by the protrusion of any part of its surface, the 

 protruded parts being known as " pseudopodia," the rest of the 

 protoplasm flowing in a wave-like manner after thesB processes. 



Inside the protoplasm of a cell is a body called the nucleus. 

 The nucleus is composed of a more liquid part, the " nuclear 

 fluid," and a more solid part, the "nuclear protoplasm." The 

 nucleus varies in form : sometimes it is round, at others oval, 

 or again it may be elongated and twisted. 



Both protoplasm and nucleus are surrounded, as a rule, by a 

 definite layer, which more nr lens retains the contractile proto- 

 plasm. The essential part of the cell is the protoplasm, which 

 has the power of independent movement, of metabolism, and 

 of reproduction. All organisms that we shall deal with, except- 

 ing the simple Protozoa, will be seen to be made up of numbers 

 of these cells, which become united in various ways, and so 

 form the animal tissues. "\Ve know that a cell always origin- 

 ates from a pre-existing cell. The formation of one cell from 

 another takes place chiefly by a process known as karyoJcinesis 

 or " cell-division." 



When a cell has received its full share of nourishment — that 

 is, when it has reached maturity — its protoplasm commences to 

 separate into two equal halves. This division is preceded by a 

 corresponding separation of the nucleus, and then the whole cell 

 splits into two cells. During this process of cell-division certain 

 definite changes take place in the nucleus. This body at first 

 is spindle-shaped ; its contents are drawn out into longitudinal 

 strife, when the centre of these stria; becomes thickened and 

 forms an equatorial zone or " nuclear plate." This " plate " then 

 divides, and each half travels to the poles of the spindle, which 

 assumes a dumb-bell shape, then elongates, and the two nuclear 



