14 INTRODUCTION 



aspects of function with relation to the structural features, although 

 this study is primarily the problem of physiology. 



The study of histology and microscopic anatomy rests upon the 

 structural and functional differentiations of the cells. Although 

 the knowledge founded on studies of the protoplasmic structure and 

 function of cells belongs to the science of cytology, it is essential 

 that the student of histology should know some of the general 

 structural features of the cell in order to understand the conditions 

 met w ith in the tissue and organ structures with which his prepara- 

 tions deal. It is also important to remember that complex multi- 

 cellular animals usually originate from the fusion of a single egg 

 and a sperm. The progressive steps by which such a fertilized egg 

 develops into a mature individual is the concern of embryology. 

 We will later follow the proliferation of cells, at first similar in 

 structure, through their early embryonic development up to the 

 time when cellular differentiation becomes more apparent and the 

 three cellular layers, the ectoderm, endoderm and mesoderm, are 

 formed. From these three layers all the later tissues and organs 

 are derived. 



The study of the living cell is possible, but it is still far from 

 being sufficiently practical to be used as a method for teaching. 

 We must rely, therefore, almost entirely upon preparations of dead 

 tissues and organs stained to accentuate their characteristic features. 

 It is true that the methods used in such preparations subject the 

 cells to a number of chemical and physical changes, but the features 

 shown by such methods have been repeatedly checked so that in 

 knowing the reaction of cells to these procedures w^e have relatively 

 constant factors with which to deal. In the living cell many of the 

 structures easily seen hi prepared material are visible only with 

 great difficulty. Although the cellular structures and intercellu- 

 lar material appearing in our preparations may be called artifacts to 

 some extent, they will gi\'e reliable information regarding char- 

 acteristic structures and reactions of tissues, providing we are 

 acquainted beforehand with the methods of preparation used. 



THE CELL. 



Although there is great variation in the size, shape, and particular 

 functions of different cells, there are certain general structural 

 features that can usually be demonstrated in all. An animal cell 

 may be defined as a small mass of protoplasm externally limited by 



