THE CELL. 



141 



c , 



spects. The nucleus has a different refractive index, so 

 that it stands out. under the microscope, like a drop 

 of oil in water. It has a great affinity for certain 

 stains, and hence in microscopical 

 preparations it is brightly col- 

 ored, while the protoplasm is 

 usually but slightly stained. 



W- N ~x 'J^jk'-a^ * \ ''"' '->C~">'-' A 



Physiological experiments show ^^fS^^ft>f5^% 



that the nucleus controls the 



action of the cell; while there 



is all but conclusive evidence that 



it includes the mechanism of 



heredity. 



In a few minute forms, both ani- 

 mals (Protozoa) and plants, the FlG 1 ._ Diagnun of a ce ii. 

 whole organism consists of but a 

 single cell, which consequently 



carries on all the functions of life, but all the rest are 

 multicellular. In the multicellular animals (Metazoa) the 

 cells are not all alike, but may be divided into groups or 

 layers specialized in different directions. This differentia- 

 tion is both of form and of function. The cells of the 

 separate layers differ in shape and have different work to 

 perform, there being what is termed a division of labor 

 among the groups of cells. 



All animals above the Protozoa reproduce by eggs. 

 These eggs, when carefully studied, are found to agree in 

 their essential characteristics. Each, in fact, is a cell 

 (p. 126) containing a nucleus; but to these essentials 

 other structures shell, white, yolk, etc. may be added. 

 Each egg, under proper conditions, is capable of growing 

 into a form like that which produced it. The essential 

 condition is that a peculiarly modified cell, the spermato- 



