LINNEAN SOCIETY OF LOXDOIf. XXI 



to the iuvestigation of the lut'usoria, Haeckel must be mentioned as 

 the one who has brought the greatest amount of evidence to bear on 

 the question of their unicellularity. In a verv elaborate paper which 

 has quite recently appeared*, and which is remarkable for the clear- 

 ness and logical acuteness with which the whole subject is treated. 

 Prof. Haeckel, resting mainly on the observations of others and 

 partly also on his own, argues in favour of the unicellularity of the 

 Infusoria from the evidence afforded both by the phenomena of their 

 development and by the structure of the mature organism. He 

 confines himself chiefly to the Ciliata (which, indeed, he regards as 

 the only true Infusoria), while he considers the unicellularity of the 

 Flagellata as too obvious to require an elaborate defence. The 

 value of this paper will be obvious from the analysis of it which 

 1 now propose to give. 



In stating the argument derived from development, Haeckel does 

 not accept as established the alleged sexual reproduction of the In- 

 fusoria ; and he believes it safest to regard as non-sexual spores the 

 bodies (Keimkugeln) which result from the breaking up of the nucleus, 

 and which Balbiani regarded as eggs. 



These bodies consist of a little mass of protoplasm usually desti- 

 tute of membrane and including a nucleus, within which one or more 

 vefringent granules, admitting of comparison with a true nucleolus, 

 may sometimes be witnessed — characters which are all those of a 

 simple genuine cell. From this spore the embryo is developed by 

 direct growth and differentiation of parts ; but however great may be 

 the differentiation, there is never any thing like the formation of a 

 tissue. 



The development of the Infusoria is thus entirely in favour of the 

 unicellular theory. This theory, however, is just as strongly sup- 

 ported by the study of their mature condition ; and here Haeckel 

 gives an admirable exposition of the structure of the true or ciliate 

 Infusoria. 



The parts which are common to all Ciliata, which first differentiate 

 themselves in the ontogenesis or development of the spore, are the 

 cortical layer, the medullary parenchyma, and the nucleus, which is 

 situated on the boundary between the two. The differentiation of 

 the protoplasm of the naked spore into a clearer and firmer cortical 

 substance, and a more turbid, granular, and softer medullary sub- 

 stance, corresponds entirely with what we see in Amoeba and the 



* Haeckel, " Zur Morphologie der InfuBorien,"Jenais3he Zeitsch, Band vii. 

 Heft 4. 1873. 



