XXVI PEOCEEBINGS OF THE 



ration and excretion. They are in all cases destitute of proper 

 walls, and they have been long recognized as, morphologically, no- 

 thing more than lacunae filled with fluid. Regular contractile ve- 

 sicles, differing in no respect from those of the ciliate Infusoria, are 

 often found in the Flagellatae and in the swarm-spores of many 

 Algse. 



Besides the constant and regularly contracting vacuoles, there 

 occur also others less constant and less regularly contracting. These 

 are found in the softer endoplasm, while the constant and regularly 

 contracting vacuoles occur for the most part in the firmer exoplasm. 

 One is just as much a wall-less vacuole as the other, and the differ- 

 ence between them is to be traced to the difference of consistence in 

 the surrounding protoplasm. Haeckel regards the less constant 

 ones as the original forms from which the others have been phylo- 

 genetically derived — that is, by a process of inheritance and modifica- 

 tion through descent. 



The last and most important of the parts which enter into the 

 formation of the Infusorium-body, namely the nucleus, is next dis- 

 cussed. Viewed from a morphological point, it has been already 

 demonstrated that the nucleus is in all Ciliata originally a single 

 simple structure, resembling in this respect a true cell-nucleus. As 

 the Infusorium-body approaches maturity, we find that, with its 

 advancing differentiation, peculiar changes occur in the nucleus, just 

 as in the rest of the protoplasm ; but these changes are entirely 

 parallelled by differentiation phenomena which are known in other 

 undoubted cell-nuclei, as, for example, in the germinal vesicle of 

 many animals, in the nuclei of many unicellular plants, the nuclei of 

 many parenchyma-cells of the higher plants, and the nuclei of many 

 nerve-cells. The mature Infusorium -nucleus is often vesicle-like, 

 and consists of a delicate investing membrane and fine granular 

 contents, precisely as in the differentiated nucleus of many other 

 cells. In many Ciliata, if not in all, there is within the young 

 nucleus a dark more refringent corpuscle, which has quite the same 

 relations as the nucleolus of a true cell-nucleus. 



Regarded from a physiological, no less than from a morphological 

 point of view, the infusorium-nucleus and true cell-nucleus admit of 

 a close comparison with one another. It may be considered as 

 established by the concurrent observations of all investigators that 

 the nucleus of the Infusoria performs the functions of a reproductive 

 organ, though the opinions entertained as to the mode in which it 

 thus acts are extremely divergent. 



