442 TBANSAOTIONS OF SECTION D. 



tered without order or arrangement throughout the protoplasmic body, is a 

 mass of chromatin or a clump of chromatin-grains supported on a framework 

 and lodged in a special vacuole in the cytoplasm. The complexity seen in the 

 most perfect type of nucleus takes origin by progressive elaborations of, and 

 additions to, a structure of this simple and primitive type. 



This brings me to a point which I wish to emphasise most strongly, namely, 

 that the conception of a true cell-nucleus is essentially a structural conception. 

 A nucleus i.s not merely an aggregation of chromatin ; it is not simply a central 

 core of some chemical substance or material differing in nature from the 

 remainder of the protoplasm. As Dobell has well expressed it, a pound of 

 chromatin would not make a rucleus. The concepts ' nucleus ' and ' chromatin ' 

 differ as do those of 'table' and 'woo<l.' Although chromatin is the one 

 universal and necessary constituent entering into the composition of the cell- 

 nucleus, a simple mass of chromatin is not a nucleus.' A true nucleus is a 

 cell-organ, of greater or less structural complexity, which has been elaborated 

 progressively in the course of the evolution of the cell ; it is as much an organ 

 of the cell as the brain is an organ of the human body. As a definite cell- 

 organ, it performs in the life and economy of the cell definite functions, which 

 it is the province of the cytologist to observe and to study, and if possible 

 to elucidate and explain. As an organ of the cell, however, it has no homo- 

 logue or analogue in the body of the multicellular animals or plants ; there is 

 no organ of the human body, taken as a whole, similar or comparable to the 

 nucleus of the cell. Consequently, in studying the functions of the nucleus 

 the human cytologist finds himself in the same difficult position that an intelli- 

 gent living being lacking the sense of sight would be when trying to discover 

 the function of visual organs in other organisms possessing that sense. There 

 is no organ of known and understood functions with which the cytologist can 

 compare the cell-nucleus directly. 



The foregoing brief consideration of the nucleus leads me now to discuss in 

 more detail the nature and properties of the essential nuclear substance, the 

 so-called chromatin. To define, or characterise adequately, this substance is a 

 difficult task. The name chromatin is derived from the fact that this sub- 

 stance has a peculiar affinity for certain dyes or stains, .so that when a cell is 

 treated with the appropriate colouring reagents — with so-called nuclear stains 

 — the chromatin in the nucleus stands out sharply, by reason of being coloured 

 in a different manner from the rest of the cell. In consequence, the state- 

 ment is frequently made, in a loose manner and without reflection, that 

 chromatin is recognised by its staining reactionrs, but in reality this is far from 

 being true. When a preparation of an ordinary cell is made by the methods 

 of technique commonly in use, the chromatin rs recognised and identified by 

 its position in a definite body with characteristic structure and relations to the 

 cell as a whole, namely the nucleus, and this is equally true whether the 

 chromatin has been stained or not. When the cell has been stained with one 

 of the dyes ordinarily in use for colouring the chromatin, there are often seen 

 in the cytoplasm grains that are coloured in exactly the same manner as the 

 chromatin-grains lodged in the nucleus. Is an extranuclear grain which stains 

 like chromatin to be identified, ipso facto, as chromatin? By no means; it 

 may or it may not be chromatin. Simple inspection of a stained preparation 

 is altogether inadequate to determine whether such a body is or is not 

 chromatin. Any so-called chromatin-stain colours many bodies which may occur 

 in a cell besides tiie chromatin, and it may be necessary to try a great many 



' Professor Armstrong writes : ' Every organism must possess some kind of 

 nucleus, visible or invisible ; some formative centre round which the various 

 templates assemble that are active in directing the growth of the organism.' 

 {Srirncfi Prof/rcx.i, vol. vii. p. 328.) I need hardly point out that a chemi<al 

 nuileu.s of this kind is not in the least what the biologist or cytologist means 

 by the term cell-nucleus. Tlie one is a subjective postulate necessary for the 

 comprehension of the activities of any speck of living matter or any portion, 

 liowever minute, of a living organism ; the other is a concrete structure, known 

 to us by actual observation, and as much an integral part of the true cell, 

 considered as a definite type of organism, as a backbone or its morphological 

 cquivnlcnt is essential to the definition of a true vertebrate. 



