34 GENERAL EMBRYOLOGY 



taming a nucleus." To-day we modify this but slightly and 

 define a cell as a limited mass of protoplasm containing nuclear 

 material. 



The mass of the cell is usually very small. The smallest 

 cells known are the Bacteria, some of which may be only 

 0.001 mm. (1 micron, or 1/25000 inch) in length (Streptococci), 

 or even less (Staphylococci). But among the Metazoa, cells 

 are never so minute. The human white blood corpuscle or 

 leucocyte (Fig. 14, 7), is perhaps of average size, measuring 

 something less than 0.01 mm. (8-10 micro) in diameter. 

 Tissue cells having a diameter of 0.05 mm. (50 micro) are 

 considered large, although a few specialized cells may far exceed 

 this (e.g., muscle or nerve cells). The egg cells of animals are 

 usually larger than tissue cells, but this is a special condition, 

 and is frequently due to the accumulation of stored food sub- 

 stance, rather than to the possession of a larger amount of 

 protoplasm. Within the species the sizes of specific varieties 

 of cells are very constant. The size of an organ or of an 

 individual is related to the number of its component cells 

 rather than to their size (Amelung, Conklin). 



We may proceed now to describe the essentials of structure 

 exhibited by a typical cell an imaginary thing which has no 

 more real existence than the " average man." Such a cell 

 would consist of a spheroidal or irregular mass of protoplasm, 

 limited by a definite cell membrane or cell wall. The wall may 

 be a surface condensation of the protoplasm or, more frequently, 

 a true secretion of the cell body, either membranous as in most 

 animal tissues, or thick and rigid, like the cellulose walls of 

 most plants. In many cells the viscid transparent protoplasm 

 just within and in contact with the cell wall forms a thin layer, 

 the ectosarc or ectoplasm (Fig. 16), clearer than the granular 

 and more refractive central endosarc or endoplasm, which 

 contains, besides the granules, many cell organs and inclusions. 



The protoplasm itself is made up of a combination of two 

 forms, perhaps two kinds, of material plainly differing in density 

 and arrangement (Fig. 17). The denser material called the 

 mitome, spongioplasm, reticulum, or filar substance, forms a sort 



