LINNEAN SOCIETY OF LONDON. XIU 



(Ciliata) and the mouthless Astomata (Flagellata) . From every 

 point of view Von Siebold's conception of the morphology of the 

 Protozoa, and his sketch of their classification, however much this 

 may have been subsequently modified, must be regarded as marking 

 out an epoch in the history of Zoology. 



Shortly after this the unicellular theory was strongly supported by 

 Kblliker *, and received further confirmation from the researches of 

 Stein f, who, however, was unable to accept it to its full extent. "With 

 an industry almost equal to that of Ehrenberg, Stein had the advantage 

 of the more philosophic views of organization which had emanated 

 from the newer schools of Biology ; and to him we are indebted 

 not only for more accurate views of the structure of the Infusoria, but 

 for the first important contributions to our knowledge of their develop- 

 ment ; and though the opinion which he at one time entertained that 

 the true AcinetcB are only stages in the development of the higher 

 Infusoria has been abandoned by him, he has^ nevertheless, demon- 

 strated the presence, in an early period of the development of certain 

 species, of peculiar pseudopodial processes resembling the charac- 

 teristic capitate appendages of the AmietcB, an observation of impor- 

 tance in its bearing on the relations of these last to the true Infu- 

 soria. No doubt can remain, after Stein's observations, that the 

 Infusoria in their young state have the morphological value of a 

 simple cell ; and it is only after their development has become ad- 

 vanced, and that a marked differentiation has begun to manifest 

 itself in this primordial condition, that there can be any difficulty in 

 accepting their absolute unicellularity. 



About this time Balbiani drew attention to some very important 

 phenomena in the life-history of the Infusoria J. It had been 

 known even to the early observers that the Infusoria multiplied 

 themselves by a process of spontaneous fission. They had been 

 frequently observed in the act of transverse cleavage, and had also 

 been noticed in what appeared to be a similar cleavage taking place 

 in a longitudinal instead of a transverse direction. Balbiani, how- 

 ever, showed that this apparent longitudinal cleavage had, in many 

 cases, an entirely different significance — that it was, in fact, not the 

 cleavage of a single individual, but the conjugation of two distinct 

 ones ; and he connected this phenomenon with what he regarded as 

 a true sexual act. 



* Zeitschr. f. wissensch. Zool. 1849. 



t Stein, ' Der Organismus der Inf usionstliiere,' 1 867. 



I Balbiani, " Recherches sur les organes generateurs et la reproduction des 

 Infuaoires," Coraptes Rondus. 1858, p. 383. 



