EXERCISE II 

 THE CELL 



It is the purpose of this exercise by means of a study of actual mate- 

 rials to acquaint the student with the general facts in regard to the 

 structure of the cell and the extent to which the cell occurs as a unit of 

 structure in living things. In order to accomplish this object a general 

 problem is stated and this general problem is subdivided into minor 

 problems. Appropriate materials with suggestions for study are given 

 under each subdivision. 



At the conclusion of the exercise the student should be able to formu- 

 late certain inferences in regard to the cell. He should realize that the 

 study of the relatively small number of materials suggested in the course 

 of this exercise do not furnish data sufficient for the confirmation of the 

 cell doctrine but that the facts observed belong to certain classes of 

 facts on which the modern cell doctrine is based. 



A. THE GENERAL PROBLEM 



To determine the structure of the cell and the extent of its occurrence 

 as a unit of structure in living things. 



1. What Structural Features are Common to Cells? 



A complete answer to this problem cannot be made until all the 

 materials in this exercise have been studied but a study of cells from 

 the four sources indicated below (la, 16, Ic, Id) will serve to introduce 

 certain structures and will give an idea as to the forms which a cell may 

 assume. 



la. Place a drop of water on a clean slide and mount in it a small 

 piece of stratum corneum of frog skin (the outermost layer that is re- 

 peatedly shed). Spread the specimen flat, and cover with a cover-glass. 

 Examine with the microscope, trying out different light intensities. 



Note the units of which the tissue is composed. These are the cells. 

 Each contains a dense mass, the nucleus (plural, nuclei), which is usually 

 visible. The remainder of the contents of the cell, besides the nucleus, 

 is to all appearances nearly structureless and is known as cytoplasm. 

 Both nucleus and cytoplasm are composed of protoplasm. The surface 

 layer of each of these cells is the cell membrane. 



Now remove the cover-glass or mount a fresh piece of stratum cor- 

 neum. Draw off the excess water with filter paper or a blotter, and add a 



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