MODERN BIOLOGY 419 



Infusoria's character of simple ceils; their nuclei, which Ehrenberg believed 

 to be sexual organs, Dujardin was unable to explain more exactly, but 

 considered them to be simply concretions in the sarcode. 



Siebold on Infusoria 

 It was reserved for Siebold to put the Infusoria in their right place. As early 

 as his Comparative Anatomy of 1845, which has been referred to above, he 

 combines Infusoria and Rhizopoda under the common term Protozoa and 

 characterizes them as ' ' Tiere, in ivelchen die verschiedenen Systeme der Organe nicht 

 scbarf aiisgeschieden sind, und dercn unregelmdssige Form tmd einjache Organisation 

 sich auj eine Zelle rediixjeren lassen.'' This definition he bases on a careful de- 

 marcation of the forms included in the group; the Rotatoria are definitely 

 separated as being more highly organized, and a number of multicellular, 

 but primitive life-forms, which produce chlorophyll — Closterines, Vol- 

 vocines — are transferred to the vegetable kingdom. It is pointed out in 

 connexion therewith that cilia- and flagella-movements can also exist in 

 the vegetable kingdom, while on the other hand a special free mobility of 

 a higher type is attributable to the Protozoa. The various organic systems 

 that Ehrenberg ascribed to the Infusoria are examined and rejected; there 

 thus remains the simple cell, provided with nucleus and vacuoles, which is 

 hereby proved to be capable of sustaining a free and independent existence, 

 being reproduced by division without any special sexual organs. 



In a paper published some years later Siebold further examines the exist- 

 ence of and the relation between single-celled animals and plants, being sup- 

 ported in his views particularly by a work by Nageli, which had then been 

 recently published, on unicellular algx, in which these organisms had been 

 thoroughly characterized and described. As a result of these works micro- 

 biology was directed on the right way and during the next-epoch came to 

 exercise a great influence on the development of biology in general. In the 

 Protozoa, "the primary animals," had been found a category of living 

 creatures from which, as the name implies, the other higher organisms 

 could be derived, besides which the cell idea was hereby made to cover an 

 entirely new area; it was possible to see in the cell not only the basic element 

 in the structure of the organisms, but also a true elementary organism capable 

 of leading an independent life or, as a transition to the higher cell-structures, 

 of forming colonies of similar elements, such as the Volvox referred to above. 

 Space forbids our going further into the maze of works which were now de- 

 voted to the single-celled animals and plants. During the succeeding period 

 in particular, research work in this field went on apace without interruption. 

 Of the works on the Protozoa that appeared during this era may be mentioned 

 those of Friedrich Stein (1818-85), professor at Prague, on the Infusoria, 

 which form the basis for all later research on the subject. 



There still remains to deal with in this connexion one more group of 



