PROTOPLASMIC STRUCTURE 



21 



FIG. 2 



Cells. Animal bodies are made up of tissues. Tissues are divided 

 into organs. Organs are composed of cells, which in turn have biological 

 divisions. At the risk of impinging, to some extent on histology, we must 

 look (but briefly) at the parts which make up cells, as an introduction to 

 the study of the composition and the properties of protoplasm. In 1665 

 (the famous year of the great London 

 plague) Robert Hook, an English nat- 

 uralist, discovered organic cells in vege- 

 table tissues. He named them cells 

 because the little "chambers" seemed 

 to him, using the rude microscopes of 

 the day (discovered only fifty years 

 before), like hollow and empty cavi- 

 ties. A dozen years later, Malpighi, 

 the Italian anatomist, recognized that 

 these elements were masses of tissue, 

 each with walls of its own, so that, after 

 all, the animal cell is a chamber, 

 although one largely filled with liquid. 



A living cell in general is made up of 

 at least four or five parts, cytoplasm, 

 nucleoplasm, or nucleus, the centrosome 

 within the attraction sphere, and some- 

 times, if not always, a limiting mem- 

 brane. The cytoplasm consists of a 

 homogeneous, semifluid, and somewhat 

 transparent mass (the enchylema), and, 

 variously arranged within this, other 

 perhaps more solid masses or lines or 

 granules of a different nature. The 

 nucleoplasm is composed of a nuclear 

 sap, the enchylema, a linin network, 

 various chromatic masses of nuclein, 

 and a nucleolus. About all these, 

 dividing it from the surrounding cyto- 

 plasm, is a nuclear membrane. The 

 centrosome and the attraction-sphere 

 containing it (concerned especially in 

 reproduction) are apparently permanent 

 organs of the cell; they are seen most 

 often and most plainly at the time of 

 the division of the cell. To some observers the centrosome appears as 

 the center of activity of the cell. 



That the cells of animals and plants have nuclei was discovered by 

 Fontana in 1781. The nucleus of a cell is usually a rounded mass, but 

 it may have other forms, as in vorticella (c-shaped) and the distributed 

 granular form of certain rhizopods. It usually constitutes but a small 



Radial section in the wood of a young 

 spruce, to show the vegetable type of cell 

 and also the circulatory mechanism of a 

 plant. (U. S. Dept. of Agriculture, 

 Farmers' Bulletin, No. 173.) 



