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The substance which is thus active is a mass of proto- 

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

 principle, from that of the protoplasm of the nettle. 

 Under sundry circumstances the corpuscle dies and 

 becomes distended 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 framework of 

 the body. Nay, more ; in the earliest condition of the 

 human organism, in that state in which it has but just 

 become distinguishable 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 aggregation. 



Thus a nucleated mass of protoplasm turns out to be 

 what may be termed the structural unit of the human 

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

 is a, mere multiple of such units ; and, in its perfect con- 

 dition, it is a multiple of such units, variously modified. 



But does the formula which expresses the essential 

 structural character of the highest animal cover all the 

 rest, as the statement of its powers and faculties covered 

 that of all others? Very nearly. Beast and fowl, 

 reptile and fish, mollusk, worm, and polype, are all com- 

 posed of structural units of the same character, namely, 

 masses of protoplasm with a nucleus. There are sundry 

 very low animals, each of which, structurally, is a mere 

 colourless blood-corpuscle, leading an independent life. 

 But, at the very bottom of the animal scale, even this 

 simplicity becomes simplified, and all the phenomena of 

 life are manifested by a particle of protoplasm without a 

 nucleus. Nor are such organisms insignificant by reason 

 of their want of complexity. It is a fair question 



