172 
Recent Progress in our Knowledge of 
great Prussian observer should still adhere with undiminished 
tenacity to his original views. 
Among the authors who, since the publication of the ‘ Infu- 
sionsthierchen,’ have contributed most to a correct estimate of the 
morphology, physiology, and systematic position of the Infusoria, 
the names of Dujardin, Yon Siebold, Stein, Balbiani, Claparede, and 
Lachmann, and most recently, Haeckel, stand out conspicuous. 
The way to a philosophic conception of the Infusoria and of 
other beings which occupy the lowest stages of life was undoubtedly 
opened up by Dujardin* when he drew attention to the existence of 
a peculiar form of matter of semi-fluid consistence and of nitro- 
genous composition, and which, though totally undifferentiated, is 
yet endowed with properties essentially characteristic of vitality. 
To this remarkable substance 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 im- 
portance 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.f Yon Siebold rejected in toto the polygastric theory, 
and, so far from admitting a complexity in the organization of 
the Infusoria, he regarded 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 
surrounded ; 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 Flagellatse — was extended by 
Siebold to Amoeba and its allies, and to the whole assemblage so 
constituted he assigned the position of a primary group of the 
* “ Sur l’organisation des Infusoires,” ‘Ann. des Sci. Nat.,’ 1838; and ‘Hist, 
des Infusoires,’ Paris, 1841. 
f Siebold, ‘Lehrbuch dor vergleichenden Anatomic,’ 1845. 
