i; 2 SCIENCE FROM AN EASY CHAIR 



of a " cell " always contains within it a special, firmer, 

 and denser part, enclosed in an enveloping coat or skin. 

 This dense body is the " nucleus," or kernel, and is of the 

 very greatest importance in the chemical changes and 

 movements which constitute the life of the cell. It is 

 usually spherical, and in the living state often looks clear 

 and bright. All cells, whether they are found building 



FIG. 37. A, cells forming soft vegetable tissue ; a, cell-wall ; b, pro- 

 toplasm ; c, liquid-holding cavity in the protoplasm ; d, the nucleus. 

 B, a pigment-cell from the frog's skin, expanded. C, the same 

 cell contracted. D, a nerve-cell : observe the nucleus. E, a muscle- 

 cell stretched. F, the same contracted : observe the nucleus. 



up the bodies of plants and animals like so many living 

 bricks, or living freely and singly as animalcules, have 

 the essential structure just described a semi-liquid yet 

 tenacious material enclosing a globular firmer body, the 

 nucleus. 



How did these viscous nucleated corpuscles come to 

 be called " cells " ? It was in this wise. At the end of 

 the seventeenth century Dr. Robert Hook, secretary of 



