INTRODUCTION 



5 



by Huxley as the " physical basis of life," and at the present time 

 universally recognized as the immediate substratum of all vital 

 activity.^ Endlessly diversified in the details of their form and struc- 

 ture, these protoplasmic masses nevertheless possess a characteristic 

 type of organization common to them all ; hence, in a certain sense, 

 they may be regarded as elementary organic units out of which the 

 body is compounded. This composite structure is, however, character- 



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cv 



Fig. 3. — A/nceba Proteus, an animal consisting of a single naked cell, x 280. (From Sedgwick 

 and Wilson's Biology.) 



n. The nucleus; w.v. water-vacuoles; c.v. contractile vacuole ; f.v. food-vacuole. 



istic of only the higher forms of life. Among the lowest forms at the 

 base of the series are an immense number of microscopic plants and 

 animals, familiar examples of which are the bacteria, diatoms, rhizo- 

 pods, and Infusoria, in which the entire body consists of a .single cell 

 (Fig. 3), of the same general type as those which in the higher multi- 

 cellular forms are associated to form one organic whole. Structurally, 

 therefore, the multicellular body is in a certain sense comparable with 

 a colony or aggregation of the lower one-celled forms.- This com- 



1 The word prcioplasm is due to Purkinje (1840), who applied it to the formative sub-, 

 stance of the animal embryo and compared it with the granular material of vegetable 

 "cambium." It was afterward independently used by H. von Mohl (1846) to designate 

 the contents of the plant-cell. The full physiological significance of protoplasm, its identity 

 with the "sarcode" (Dujardin) of the unicellular forms, and its essential similarity in 

 plants and animals, was first clearly placed in evidence through the classical works of Max 

 Schultze and De Bary, beside which should be placed the earlier works of Dujardin, Unger, 

 Nageli, and Mohl, and that of Cohn, Huxley, Virchow, Leydig, Briicke, Kiihne, and Beale. 



2 This comparison must be taken with some reservation, as will appear beyond. 



