10 ANIMAL STUDIES 



as in the case of the hen's or ostrich's egg. On the average 

 a cell will measure between .025 to .031 mm. ( T oVo an ^ 

 ^J-o of an inch) in diameter, a speck probably invisible to 

 the unaided eye. While the size and external appearance 

 of a cell are seen to be most variable, the internal structures 

 are found to show a striking resemblance throughout. All 

 are constructed upon essentially the same plan. Differ- 

 ences in form and size are superficial, and in passing to a 

 more careful study of one cell we gain a knowledge of the 

 important features of all. 



11. A typical cell. An egg-cell (Fig. 1, A) or some sim- 

 ple one from the liver or skin may be chosen as a good rep- 

 resentative of a typical cell. To the naked eye it is barely 

 visible as a minute speck ; but under the microscope the 

 appearance is that of so much white of egg, an almost trans- 

 parent jelly-like mass bearing upon its outer surface a thin 

 structureless membrane that serves to preserve its general 

 shape and also to protect the delicate cell material within. 

 The comparison of the latter substance to egg albumen can 

 be carried no further than the simple physical appearance, 

 for albumen belongs to that great class of substances which 

 are said to be non-living or dead, while the cell material 

 or protoplasm, as it is termed, is a living substance. "We 

 know of no case where life exists apart from protoplasm, 

 and for this reason the latter is frequently termed the 

 physical basis of life. 



In addition to the features already described, the proto- 

 plasm of every perfect cell is modified upon the interior to 

 form a well-defined spherical mass known as the nucleus. 

 Other structures are known to occur in the typical cell. 

 Experiment shows that the nucleus and cell protoplasm are 

 absolutely indispensable, whatever their size and shape, and 

 therefore we are at present justified in defining the cell as 

 a small mass of protoplasm enclosing a nucleus. 



12. Structure of protoplasm. When seen under a glass 

 of moderate power protoplasm gives no indication of any 



