NUCLEOLAR STRUCTURES OF THE CELL. 167 



secretion of the chromatin, and that little, if any, chromatin 

 substance enters into its composition. 



That the chemical nature of nucleoli varies in different cells 

 is very probable; and in such objects as germinal vesicles of 

 insects the component granules of a nucleolus appear quite 

 heterogeneous in nature. These cases we may explain on 

 Rhumbler's theory, that compound nucleoli are formed by 

 consecutive fusions of smaller nucleoli, the latter varying 

 physically and chemically, since they are produced at dif- 

 ferent times. 



Such appears to me to be the genesis of nucleolar substance. 

 But is there any probability that a nucleolus at some stage in its 

 history may become an organ, i.e., actively fulfil a particular func- 

 tion } It has been considered by Hacker as an excretory organ, 

 contractile vacuoles in it having been observed by him and Bal- 

 biani ; while van Beneden has described rhythmic expansion and 

 contraction of a nucleolus. It seems to me probable, but by no 

 means definitely settled, that in most cases the nucleolus can- 

 not be considered more than an inert mass ; and that if it should 

 in any case prove to become differentiated, or vitalized, into an 

 organ, it could be regarded as little more than a receptacle for 

 waste products of the nucleus, possibly exerting a force to attract 

 such substances to itself. But the mass of evidence would show 

 the nucleolus to be a mass of non-living substance. The usually 

 globular form can well be explained upon its usual fluid consist- 

 ency ; and the regularity in number, which is a striking phe- 

 nomenon in certain nuclei, as, e.g., the pronuclei of fecundation, 

 may well be explained by the assumption that there are partic- 

 ular spaces within the nucleus specially fitted for the deposition 

 of nucleolar substance. 



The hypothesis of Julin' and Henneguy, that the nucleolus 

 and Balbianian corpuscle of metazoan ova together correspond 

 to the macronucleus of Infusoria is interesting, but an interest- 

 ing speculation only. It is of importance to note that in this 

 group of Protozoa only the vegetative macronuclei contain true 

 nucleoli, while they are absent in the animal micronuclei. 



Here may be mentioned briefly the conclusions of certain 

 authors in regard to the nature of the nucleolus, in order to 



