440 TRANSACTIONS OF SECTION D. 



walls, and containing a relatively large quantity of fluid cell-sap and a small 

 quantity of the slimy protoplasmic substance. When these structures were first 

 discovered, the limiting membrane or wall of the cell was regarded as essential, 

 and less importance was attached to its contents. With increased knowledge, 

 however, and especially when animal tissues came to be studied, it became 

 apparent that the cell-wall, like the fluid cell-sap, was a secondary product, and 

 that the essential and primary part of the cell was the viscid protoplasmic sub- 

 stance, in which a peculiar body, the ' nucleus ' or kernel, was found to be uni- 

 versally present. Consequently the application and meaning of the term cell had 

 to undergo an entire change, and it was defined as a small mass or corpuscle of 

 the living substance, protoplasm, containing at least one nucleus. To these 

 essential constituents other structures, such as a limiting membrane or cell- 

 wall, and internal spaces — vacuoles — filled with watery fluid, might be added 

 as products of the secretory or formative activity of the living substance; 

 but such structures were no longer regarded as essential to the definition of the 

 cell, since in many cases they are not present. It is to be regretted in some 

 respects that with this changed point of view the term ' cell,' used originally 

 under a misapprehension, was not replaced by some other term of which 

 the ordinary significance would have been more applicable to the body denoted 

 by it." 



The chief point that I wish to establish, however, is that the term cell was 

 applied originally to the protoplasmic corpuscles building up the bodies of the 

 Metazoa and Metaphyta, each such corpuscle consisting of a minute indi- 

 vidualised mass of the living substance and containing a nucleus. Hence a 

 complete cell is made up of two principal parts or regions, the nucleus and the 

 remainder of the protoplasmic body, termed the cytoplasm. By some authors 

 the term protoplasm is restricted to the cytoplasmic portion of the cell, and proto- 

 plasm is then contrasted with nucleus ; but it is more convenient to consider 

 the whole cell as composed of protoplasm divided into two regions, nucleus and 

 cytoplasm. 



We come now to the consideration of the body termed the nucleus, which 

 undoubtedly possesses an importance in the life and functions of the cell far 

 greater than would be inferred from the name given to it. A nucleus, as seen 

 in its typical form, has a limiting membrane enclosing a framework composed 

 of a substance termed ' linin.' The framework has the form of a network, 

 which is probably to be interpreted, primitively at least, as the optical expres- 

 sion of an alveolar structure similar to that seen also in the cyioplasm, but of 

 coarser texture, and the apparent ' threads ' of the linin-framework may then 

 he the optical sections of the partitions between neighbouring alveoli. Such an 

 interpretation does not exclude the possibility of the formation of real thread.'? 

 or fibres in the framework in certain cases or during particular periods of 

 nuclear activity ; just as fibrous structures may arise in the alveolar cytoplasm 

 also. The cavities of the framework contain a watery fluid or nuclear sap, 

 probably of the same nature as the fluid enchylema or cell-sap contained in the 

 alveolar framework of the cytoplasm. At the nodes of the alveolar framework 

 are lodged grains or masses of chromatin, a substance which must engage our 

 most particular attention, since it is the essential constituent of the nucleus, 

 universally present in all nuclei, whether of the simplest or of the most complex 

 types. In addition to the chromatin -grains, which are distributed in various 

 ways over the linin-framework, there are to be found usually one or more 

 masses termed nucleoli, composed of a material which differs from chromatin in 

 its reactions and has been termed plastin. 



In the foregoing paragraph I have described in general terms the typical 

 nucleus of the text-books, as foiuid commonly in the cells that build up the 

 bodies of ordinary animals and plants. The minutife of the details of structure 

 and arrangement of the constituent parts may vary infinitely, but the type 

 remains fairly constant. When we come, however, to the nuclei of the Pro- 

 tista, such pronounced modifications and variations of the type are met with 



" 'Nothing could be less appropriate than to call such a body a "cell"; 

 yet the word has become so firmly established that every effort to replace it by 

 a better has failed, and it probably must be accepted as part of the established 

 nomenclature of science.' — P]. B. Wilson, The Cell, p. 19. 



