74 CYTOPLASM, NUCLEUS, ETC. [CH. 



We have here a number of more or less cuboidal or 

 polygonal cells, each of which is bounded by a thin 

 cell-wall, and is densely filled with granular, semi-trans- 

 lucent contents in which several kinds of enclosures 

 occur. The contents consist of living protoplasm and 

 its enclosures, and since this particular mass of proto- 

 plasm constitutes the contents of a cell, we call it the 

 cell-protoplasm, or, more shortly, the Cytoplasm. 



Embedded in the cytoplasm are several objects. Some 

 of these are mere granules or delicate fibrils to all 

 appearance, while others are much more conspicuous 

 and complex living bodies, which since they never 

 occur outside living protoplasm we must regard as or- 

 ganised parts i.e. small organs or organites of the 

 cytoplasm. 



The most conspicuous of these is a large rounded body 

 called the Nucleus : it also consists of a kind of protoplasm 

 and has granules and fibrils in it, and is sharply marked 

 off from the cytoplasm by a definite boundary. It also 

 contains a dense bright rounded mass known as the 

 Nucleolus. 



In the cytoplasm, but outside the nucleus, are several 

 more or less rounded colourless bodies termed Plastidia, 

 or, shortly, plastids ; and in some cases the best ob- 

 servations show a number of other minute granule- 

 like structures lying either in the nucleus, or in the 

 cytoplasm, the most important of which are the brilliant 

 green bodies, especially abundant in leaves, called Ghloro- 

 phyl I -corpuscles. 



One of the most important of all the important 

 generalisations of the biology of the last twenty-five 

 years has been to show that, as regards essential features, 

 the constitution of a typical animal cell is like that of 

 a typical vegetable cell : there also zoologists have 



