22 PROTOPLASM AND THE CELL. 



(1860) is generally assigned the credit of having finally placed 

 this conclusion upon a secure basis; and by him the meaning of 

 the word Protoplasm was so extended as to include all living 

 matter, whether animal or vegetal. In this sense the word is 

 now universally employed. 



Appearance and Structure. Protoplasm and cells differ 

 greatly in appearance in different plants and animals, as well as 

 in different parts and different stages of development of the 

 same individual. The appearance of protoplasm and the consti- 

 tution of the cell are as a rule 



m ~----^/<x<vT^ most easily made out in very 



young structures, such as the 

 eggs of some animals or in 

 the cells of young vegetal 

 shoots. The egg of the star- 

 fish, for example, (Fig. 12), is 

 a single isolated cell of nearly 

 typical form and structure. 

 It is a minute, nearly spheri- 



Fio. 12.-Slightly diagrammatic figure of ca ] ^ O dy (_L. inch diameter) 

 the egg or ovum of a star-fish, showing the . , . , , 



structure of a typical cell, m, membrane; m wlllCJl tlirCC parts may be 

 n, nucleus ; p, protoplasm (cytoplasm). distinguished, VIZ.: (1) the 



cell-body, which forms the bulk of the cell ; (2) the nucleus, a 

 rounded vesicular body suspended in the cell-body ; (3) the mem- 

 brane or cell-wall, which immediately surrounds the cell-body. 

 Of these three, the nucleus and cell- body are mainly composed 

 of protoplasm, while the membrane is a lifeless deposit upon the 

 exterior. The protoplasm of the cell-body is generally called 

 cell-plasm, or cytoplasm, that of the nucleus nudeoplasm; that 

 is, the living matter of the cell is differentiated into two different 

 but closely related forms of protoplasm, cytoplasm and nucleo- 

 plasm. 



The Cytoplasm appears as a clear semifluid or viscid sub- 

 stance, containing numerous minute granules and of a watery 

 appearance, though it shows no tendency to mix with water. 

 Under very high powers of the microscope, especially after treat- 

 ment with suitable reagents, the clear substance is found to have 

 a definite structure, the precise nature of which is in dispute. 

 By some observers it is described as a fibrous meshwork or retic- 



