GENERAL ANATOMY 51 



I. THE MORPHOLOGICAL UNITS OF THE ANIMAL BODY. 



The Cell. The study of the morphological units of the organic 

 body first found a firm foundation in the cell theory. Every scientific study 

 of the anatomy of plants and animals must therefore take the cell as its 

 starting-point. 



The Cell Theory. In order clearly to understand the conception of 

 the cell and its name it is necessary to follow a little of the history of the 

 theory of the cell. When the name was first given to the structures in 

 plants it implied small chambers with firm walls and filled with air or 

 fluid. Then came the discovery of a small body, the nucleus, inside the 

 cell. Next Schleiden made the generalization that the cell was the ana- 

 tomical and physiological unit from which all plants are formed, but he 

 held the erroneous view that in the development of cells, the nucleus was 

 formed in a sort of matrix, then around it a nuclear membrane arose 

 by precipitation, and then a larger membrane, the cell wall, around the 

 whole. Then Schwann extended the generalization to animals, thus 

 giving it an extension to all organisms. 



In this Schleiden- Schwann cell theory the cell wall was all important, 

 as through it diffusion currents must pass between the contents and the 

 surrounding medium. Hence the wall and the contents must determine 

 the character of the cell according to physical laws. Since the life of an 

 organism is but the totality of the life of the cells of which it is composed, 

 it was thought that the theory was a great advance in the problem of the 

 physical explanation of the phenomena of life, and the origin of the cells 

 themselves was as well explained as the formation of a crystal. 



Our conception of a cell has completely changed. We know that they 

 do not arise like crystals, but from preexisting cells. The cell is not 

 merely a part of an organism; it is a physiological whole, which shows us 

 all the enigmas of life. The membrane and cell sap, so important in the 

 Schleiden-Schwann theory, have but a subordinate place, but all impor- 

 tant is the previously disregarded substance, protoplasm. Now a cell 

 may be defined as a small mass of protoplasm with one or more nuclei. 

 This change in the conception came so gradually, that the name cell has 

 persisted, although it is an evident contradiction to call a solid lump 

 without a membrane a cell. 



These changes were due to many different lines of investigation. Thus 

 the early discovery that the chlorophyl bodies in plant cells move and, later, 

 that the motion is due, not to the bodies, but to the substance in which 

 they are imbedded. This substance, to which the name protoplasm was 

 given, acquired prominence when it was found that in the simplest 



