xii PJIOCEEDINGS OF THE 



of matter of semifluid consistence and of nitrogenous composition, 

 and which, though totally undifferentiated, is yet endowed with pro- 

 perties essentially characteristic of vitality. To this remarkable sub- 

 stance he gave the name of " sarcode." The sarcode of Dujardin has 

 of late years been described chiefly under the name of " protoplasm," 

 and its wide extension and importance in the economy of all living 

 beings, whether plants or animals, has been recognized as one of the 

 most comprehensive facts in biology. 



After Dujardin, the first who, from a strong position, offered battle to 

 the authority of Ehrenberg was Carl Theodor von Siebold*. Von 

 Siebold rejected in toto the polygastric theory, and so far from 

 admitting a complexity in the organization of the Infusoria, he re- 

 garded them as realizing the conception of almost the very simplest 

 form of life, and attributed to them the morphological value of a 

 cell. 



Let us see what is involved in this most significant comparison. 

 The essential conception of a cell is, as you know, that of a more or 

 less spherical mass of protoplasm, with or without an external 

 bounding membrane, and with an internal nucleus or differentiated 

 and more or less condensed portion of the protoplasm. It was to a 

 form of this kind that Siebold compared the body of an Infusorium. 

 He called attention to the soft protoplasmic mass of which the body 

 mainly consists, to the external firmer layer by which this is sur- 

 rounded, and to the variously shaped body differentiated in the 

 protoplasm, to which Ehrenberg had gratuitously attributed the 

 function of a male generative organ. Here then were, according to 

 Siebold, the protoplasm body-substance, the bounding membrane, 

 and the nucleus of a true cell. 



The morphological value thus attributed to the true Infusoria 

 (under which were included the Flagellata) was extended by Siebold 

 to Amceba and its allies ; and to the whole assemblage thus consti- 

 tuted he assigned the position of a primary group of the animal 

 kingdom to which he gave the name of Protozoa, whose essential 

 character was thus that of being unicellular animals. He then 

 divided his Protozoa into those which had the faculty of emitting 

 pseudopodial prolongations of their protoplasm (Amceba, &c.) and 

 those in which the place of the pseudopodia was taken by vibratile 

 cilia or by lash-like appendages. To the former he gave the name 

 of Rhizopoda ; to the latter he restricted that of Infusoria ; and, 

 lastly, he divided the Infusoria into the mouth-bearing Stomatoda 

 * Siebold, ' Lehrbucli cler vergleichendeu Anatomic,' 1845. 



