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Recent Progress in our Knowledge of 
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, hut the conjugation of two distinct 
ones ; and he connected this phenomenon with what he regarded as 
a true sexual act. 
It was then known that besides the nucleus which occupied a 
conspicuous position in the protoplasmic mass, there existed in 
many Infusoria another differentiated body similar to the nucleus 
hut smaller, and either in close contact with it or separated from it 
by a greater or less interval. To this body the ill-chosen name of 
“ nucleolus ” had been given. Now, Balbiani’s observations led him 
to believe that under the influence of conjugation this so-called 
nucleolus underwent a change and developed in its interior a 
multitude of exceedingly minute filaments or rod-like bodies, to 
which he attributed the significance of spermatozoa ; while at the 
same time the nucleus became divided into globular masses, which 
Balbiani regarded as eggs, and in which he believed he could 
recognize a germinal vesicle and germinal spot. We should thus, 
according to this interpretation, have in the Infusoria the two 
essential elements of sexual differentiation, the spermatozoa and 
the egg. 
Stein, though differing from Balbiani in certain details, accepts 
in its general facts the sexual theory, and maintains the spermatic 
nature of the rod-like corpuscles to which the nucleolus appears to 
give rise. But however real may he the phenomena described by 
Balbiani and by Stein, the correctness of assigning to them a 
sexual significance may he called in question ; and it is certain that 
subsequent observation has not tended to confirm the hypothesis 
that we have in the Infusoria true eggs fecundated by true sperma- 
tozoa. 
Claparede and Lachmann, two able and indefatigable observers 
fresh from the school of the great anatomist Johannes Muller, now 
entered the field, and their joint labours have given us a valuable work 
on the Infusoria.* In this an entirely new view of the morphology 
of the Infusoria has been introduced. Receding widely from the 
unicellular theory of Siebold, they approximate towards- the views 
of Ehrenherg in assigning to the Infusoria a comparatively complex 
structure; but instead of adopting the polygastric theory of the 
Prussian microscopist, they attribute to the Infusoria a single well- 
defined gastric cavity occupying the whole of the space limited 
externally by the outer firm boundary walls of the softer proto- 
plasmic mass ; while this mass is regarded by them as nothing 
more than a sort of chyme by which the gastric cavity is filled. 
According to this view, the nearest relations of the Infusoria would 
* Claparede et Lachmann, ‘Etudes sur les Infusoires et les Rhizopodes.’ 
Geneve, 1858-61. 
