GENERAL HISTOLOGY: CELLS AND TISSUES 25 



The student will now understand that owing to the numerous processes and 

 chemical agents to which the material is subjected in the making of a slide, 

 artificial and abnormal appearances are frequently produced. Further, a differ- 

 ence in the angle at which the material is sliced will make two slides of the same 

 material present a different appearance. In studying slides, the student must 

 always bear these points in mind, and should direct his attention only to those 

 portions of the sections which are pronounced by the assistant to be typical and 

 normal, and illustrative of the structures under consideration. 



A. STUDY OF A TYPICAL CELL 



i. Liver cells of Necturus. (Necturus is an amphibian, a relative of the 

 frog, and often chosen for microscopic preparations because its cells are much 

 larger than those of the frog.) Examine with the low power the slide marked 

 " Necturus liver" and note that the liver is composed of polygonal blocks. Each 

 of the blocks is a cell, and we thus see that the liver is made up of such cells. 

 Inspection of all parts of the animal would show that they, too, are similarly 

 constructed. 



Turn on the high power and examine the structure of a liver cell in detail 

 (Hegner, pp. 26-29). Note: 



a) The cell wall, or cell membrane, the delicate partition which separates each 

 cell from its neighbors. In many cases, the cell walls may not be as distinct 

 as they are here. 



b) The nucleus, the spherical deeply stained body in the center of the cell. 

 A membrane, the nuclear membrane, separates the nucleus from the surrounding 

 cytoplasm. Within the nucleus the solid material takes the form of a network, 

 the linin network, not very distinct here, on the fibers of which are strung the 

 conspicuous, deeply stained irregular masses, the chromatin granules. This 

 chromatin is a substance characteristic of and found with few exceptions only 

 in the nucleus, and is recognizable by its staining properties. The meshes of 

 the linin network are filled with a clear, transparent, invisible fluid, called the 

 nuclear sap, the nucleoplasm, or karyolymph. 



c) The cytoplasm, the portion of the protoplasm outside of the nucleus. It 

 consists of a mixture of solid and somewhat fluid materials; the fluid portion is 

 transparent and is known as the ground substance, or cell sap, or hyaloplasm. The 

 solid material in these liver cells is apt to appear as a network, the apparent 

 fibers of which are really rows of granules. This network in the cytoplasm is 

 called the spongioplasm, and was formerly thought to represent the real structure 

 of the cytoplasm; but it is now believed to be due to the action of killing fluids 

 upon the protoplasm. Besides the spongioplasmic network, the cytoplasm 

 frequently contains granules, droplets, fibers, etc. 

 Draw a cell under high power, showing all details. 



