the Ciliaie Infusoria. By G. J. Allman. 173 
animal kingdom, to which he gave the name of Peotozoa, 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 (Amoeba, 
&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 Bhizojpoda ; to the latter he restricted that of Infu- 
soria ; and lastly, he divided the Infusoria into the mouth-bearing, 
Stomatoda (Ciliata), and the mouthless, Astomata (Flagellata). 
From every point of view Yon Siebold's conception of the mor- 
phology of the Protozoa, and his sketch of their classification, how- 
ever 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 KoUiker,* and received further confirmation from the researches 
of Stein,t 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 know- 
ledge of their development ; and though the opinion which he at 
one time entertained, that the true Acinetse are only stages in the 
development of the higher Infusoria, has been abandoned by him, 
he has nevertheless demonstrated the presence in an early period of 
the development of certain species, of peculiar pseudopodial pro- 
cesses resembling the characteristic capitate appendages of the 
Acinetse, an observation of importance in its bearing on the rela- 
tions of these last to the true Infusoria. 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 advanced, and that a marked difie- 
rentiation has begun to manifest itself in this primordial condition, 
that there can be any difficulty in accepting their absolute uni- 
cellularity. 
About this time Balbiani drew attention to some very important 
phenomena in the life history of the Infusoria.:]: 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- 
* Zeitschr. f. Wissens. Zool., 1849. 
t Stein, ' Der Organismus der Infusionsthiere,' 1867. 
X Balbiani, " Recherches ' sur les organes gcne'rateurs et la reproduction des 
Infusoires," ' Comptes Rendus,' 1858, p. 383. 
