STRUCTURE OF CELLS. 23 



cells, each endowed with all the attributes of vitality and 

 capable of complete existence singly and by itself. They are 

 typical cells. Of this kind are (a) the single cells of which 

 some of the lowest (unicellular) forms of living beings con- 

 sist, as the bacteria, desmids, and diatoms among plants ; and 

 the protozoa amrebse, foraminifera, infusoria among animals ; 

 (6) the ova and spores from which all the higher organisms 

 develop, each being an independent and undifferentiated cell, 

 and derived from the union of two other free though not typi- 

 cal cells, the female ovule and the male sperm-cell ; (c) cer- 

 tain actively vital cells living free in the fluids and tissues of 

 the highest organisms, exemplified by the white blood-corpuscle, 

 or leukocyte. 



2. Differentiated or specialized cells. In unicellular animals 

 and plants the single cell carries on all the operations of life. 

 In the forms that through evolution have become more highly 

 developed, instead of a single cell the organism is composed 

 of many cells ; and instead of each cell in the multicellular 

 organism carrying on all the functions of life, those operations 

 are divided among different sets of cells, some serving one 

 purpose, some another. This is differentiation, or specializa- 

 tion ; it is a true division or organization of labor, and by it a 

 maximum result is accomplished with the minimum and most 

 economical expenditure of energy. In such a differentiated 

 organism the cells in various situations and devoted to various 

 purposes vary accordingly in form, structure, and function. 

 Each great class of plant and animal forms has its own char- 

 acteristic types of cells. 



The specialized fixed cells of mammalian tissues may be in 

 general divided into four classes : epithelial cells, connective- 

 tissue cells, nerve-cells, and muscle-cells. 



It is worthy of note that the young or embryonal forms of 

 cells that are specialized when mature often partake of the 

 more generalized character, thus exemplifying the principle 

 that the different phases through which an organism passes 

 in the course of its genesis and development are indicative 

 of the different forms through which its species passed in the 

 course of its evolution. 



Structure of cells : All cells have a body of protoplasm, 

 which in most cases contains a nucleus (Fig. 1). The centra- 



