ORGANIZATION 22Q 



Cells vary greatly in size, but are generally invisible 

 to the naked eye, ranging from ^J 7 to IQTFO f an mcn 

 in diameter. About 4000 of the smallest would be 

 necessary to cover the dot of this letter i. The natural 

 form of isolated cells is spherical ; but when they crowd 

 each other, as seen in bone, cartilage, and muscle, their 

 outlines become angular, either hexagonal or irregular. 



Within the narrow boundary of a simple sphere, the 

 cell membrane, are exhibited all the essential phenomena 

 of life, nutrition, sensation, development, and repro- 

 duction. The physiology of these minute organisms is 

 of peculiar interest, since all animal structure is but the 

 multiplication of the cell as a unit, and the whole life of 

 an animal is that of the cells which compose it : in them 

 and by them all its vital processes are carried on. 70 



The structure of an animal cell can be seen in blood 

 corpuscles, by diluting with a weak (.6 per cent) solu- 

 tion of salt a drop of blood from a frog, and placing it 

 under the microscope. (See Fig. 260.) With this may 

 be compared vegetable cells as seen in a drop of fluid 

 yeast or a drop of water into which pollen grains from 

 some flower have been dusted. 



2. Tissues. There are organisms of the lowest 

 grade (as Paramecium, Fig. 9) which consist of a single 

 cell, living for and by itself. In this case, the animal 

 and cell are identical : the Paramecium is as truly an 

 individual as the elephant. But all animajs, save these 

 unicellular beings, are mainly aggregations of cells ; for 

 the various parts of a body are not only separable by 

 the knife into bones, muscles, nerves, etc., but these are 

 susceptible of a finer analysis by the microscope, which 

 shows that they arise from the development and union 

 of cells. These cellular fabrics, called tissties, differ 

 fro/n one another both chemically and structurally, but 

 agree in being permeable to liquids a property which 



