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precautions and under a sufficiently high microscopic 

 power, there will be seen, among the innumerable mul- 

 titude of little, circular, discoidal bodies, or corpuscles, 

 which float in it and give it its color, a comparatively 

 small number of colorless corpuscles, of somewhat lar- 

 ger size and very irregular shape. If the drop of blood 

 be kept at the temperature of the body, these colorless 

 corpuscles will be seen to exhibit a marvelous activity, 

 changing their forms with great rapidity, drawing in and 

 thrusting out prolongations of their substance, and creep- 

 ing about as if they were independent organisms. The 

 substance which is thus active is a mass of protoplasm, 

 and its activity differs in detail, rather than in principle, 

 from that of the protoplasm of the nettle. Under sun- 

 dry circumstances the corpuscle dies and becomes dis- 

 tended into a round mass, in the midst of which is seen 

 a smaller spherical body, which existed, but was more or 

 less hidden, in the living corpuscle, and is called its 

 nucleus. Corpuscles of essentially similar structure are 

 to be found in the skin, in the lining of the mouth, and 

 scattered through the whole frame work of the body. 

 Nay, more ; in the earliest condition of the human or- 

 ganism, in that state in which it has just become distin- 

 guishable from the egg in which it arises, it is nothing 

 but an aggregation of such corpuscles, and every organ 

 of the body, was, once, -no more than such an aggrega- 

 tion. Thus a nucleated mass of protoplasm turns out 

 to be what may be termed the structural unit of the hu- 

 man body. As a matter of fact, the body, in its earliest 

 state, is a mere multiple of such units ; and, in its per- 

 fect condition, it is a multiple of such units, variously 



