m. GENERAL HISTOLOGY: CELLS AND TISSUES 



The protoplasm of which living bodies are composed does not exist as a con- 

 tinuous mass, but is divided up into minute portions, each of which is called a 

 cell. A cell is defined as a small mass of protoplasm, containing a differentiated 

 body, the nucleus. All organisms are either composed of a number of cells or 

 consist of a single cell. That branch of biology which devotes itself to the 

 detailed study of the different kinds of cells which occur in living things is called 

 histology ', and that part of histology which is concerned with the structure of the 

 protoplasm hi different kinds of cells is sometimes distinguished as cytology. 



As most cells are of minute size, the microscope is necessary for their study. 

 Read carefully the sheet of instructions regarding the use of the microscope, 

 and consult the assistants on any points which you do not understand. Do not 

 fail to heed the directions regarding adjustment of the light and use of the adjust- 

 ment screws. Set up your microscope ready for use, take from your box of 

 slides the slide labeled "Necturus liver," and practice with this slide until you 

 are familiar with the method of operation of the instrument. 



NOTE. -As students frequently exhibit curiosity about the preparation of 

 microscopic slides, such as are to be used in this section of the work, a word 

 about this process may be introduced here. The piece of material of which 

 slides are desired is removed from a freshly killed animal, placed in a fluid which 

 Hlls it and preserves it in a nearly natural condition, hardened and dehydrated 

 in alcohols of increasing concentration, and imbedded in some substance such 

 as paraffin, which can be obtained in both liquid and solid condition. The 

 paraffin containing the object is then hardened in a cold medium, cut into a 

 rectangular shape and mounted on a machine called a microtome. In the 

 microtome the paraffin block is moved up and down by means of an automatic 

 micrometer screw across a very sharp knife edge, which slices off exceedingly 

 thin sections of the material which is imbedded in the paraffin. Each such slice 

 is called a section, and when these sections are mounted on the slide in the order 

 in which they are cut, they are called serial sections. While such sections may 

 be cut as thin as nnnr of a millimeter, they are generally about y-J-g- of a milli- 

 meter. After the sections are mounted on the slide in such a way that they 

 stick tightly, the paraffin is dissolved, and the piece of material is then stained 

 (in order to make the structures clearer) by dyes which are similar to those used 

 in dyeing cloth. Finally the sections are covered with a cover glass with the 

 aid of some medium which cannot dry up. 



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