46 THE CELL 



termined. It is quite possible that the structure even of undifferentiated pro- 

 toplasm or protoplasm proper, such as is found in early embr>'onal cells, is 

 not entirely uniform. The protoplasm of the more highly specialized cells 

 certainly differs markedly in structure and somewhat in chemical composition 

 in different cells. It even differs in the same cell under different functional 

 conditions. These dift'erences are apparently due to special development of 

 the protoplasm for its peculiar functions. Thus, for example, in the muscle 

 cell and the nerve cell most of what in the embryonal cell was undift'erentiated 

 protoplasm has become differentiated into contractile or conductile fibers. The 

 nucleus and a small amount of undifferentiated or less differentiated protoplasm 

 remain and are probably largely active in maintaining the nutrition of the cell. 

 Such changes in the protoplasm are of course permanent. Temporary or periodic 

 changes in the structure of protoplasm are seen in such cells as the secreting 

 cells of the pancreas or of the salivary glands where marked differences in the 

 protoplasm occur, dependent upon whether the cell is in a resting, or actively 

 secreting condition. 



Protoplasm is thus probably best considered as the material basis 

 of cell acti\dty, i.e., of Hfe, rather than as a substance having fixed 

 and definite chemical or morphological characteristics. 



It is convenient to use the term protoplasm to mean the entire 

 substance of the cell, karyoplasm to designate the protoplasm of 

 the nucleus, and cytoplasm the protoplasm of the cell body exclusive 

 of the nucleus. 



Peculiar bodies known as plastids (Fig. i) are of frequent occur- 

 rence in vegetable cells, and are also found in some animal cells. 

 They are apparently to be regarded as a differentiation of the cyto- 

 plasm, but possess a remarkable degree of independence, being 

 capable of subdi\'ision and in some cases of existence outside of 

 the cell. 



In addition to the granules which are apparently an integral part 

 of the protoplasmic structure, other granules and various cell "inclu- 

 sions" occur, to which the term metaplasm {paraplasm, deutoplasfu) 

 granules, has been applied (Fig. i). Some of these are intimately 

 associated with the cell acti\dties and represent either food substances 

 in process of being built up into the protoplasm of the cell or waste 

 products of cell metabolism. Others, such e.g., as the glycogen 

 granules of the liver cell or the mucous granules of the mucous cell, 

 are specific secretion products. Still others are fat droplets, pigment 

 granules, and various excrementitious substances. 



When the protoplasm of a cell can be differentiated into a cen- 

 tral granular area and a peripheral clear area, the former is known 

 as endoplasm,^ the latter as exoplasm. When the exoplasm forms a 



