IV. 



The Nucleolus. 



'pHE gradual improvement in our optical and chemical means of 

 i research has allowed us to peer ever deeper and deeper into 

 that anatomy of plants and animals which lies beyond the reach of 

 our unaided sight. First, it was the cell, consisting primarily of a 

 wall, and, secondarily, of "contents," which was revealed to us. 

 Later, the cell-contents were endowed with importance at the 

 expense of the wall, and were shown to consist essentially of proto- 

 plasm and nucleus ; at the present day we attempt to push the limits 

 of our knowledge further still, and speak of the finer structure of 

 protoplasm and of the compound nature of the nucleus. We have 

 learnt, thanks to the diligent and patient labours of several observers, 

 that the nucleus consists of many parts. It is bounded on the out- 

 side, m some cases at any rate, by a membrane; intertwined 

 throughout its space is a fine threadwork of a substance chemically 

 known as " linin," which is distinguished by the extreme 

 difficulty with which it stains ; embedded within the substance of 

 these threads are a greater or less number of deeply staining granules 

 —the chromatin or nuclein-grains ; the interstices of this nuclear 

 filament are occupied by a ground-substance of uncertain composi- 

 tion ; and, lastly, there occur in most nuclei one or more spherules 

 or variously shaped grains— the nucleoli— with properties peculiar to 

 themselves. It is with these last-named bodies that the present 

 article is concerned. 



In the^ first place, it must be mentioned that the term 

 " nucleolus," like so many other names occurring in the study of the 

 cell, has come to be applied to several things of an entirely dissimilar 

 nature. 



Carnoy has classified the nucleoli as they are found in the 

 literature of the cell in the following manner :— 



(i) Nucleoles iiucleiuiens, which are merely granules of nuclein, 

 either free or in connection with the nuclear threadwork. 



(2) Nucleoles noyaux, which form, as it were, a nucleus within the 

 nucleus, being in fact miniature nuclei possessed of all the usual parts, 

 embedded in a substance extremely like, if not identical with, proto- 

 plasm, and occupying the rest of the nuclear space. These peculiar 

 structures have been described in Rhizopoda and other animals. 



o 



