No. 589] THE EVOLUTION OF THE CELL 15 



mother cell. 7 The simplest body which can be recognized 

 as a nucleus, distinct from the chromidia scattered with- 

 out 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 emphasize 

 most strongly, namely, that the conception of a true cell- 

 nucleus is essentially a structural conception. A nucleus 

 is 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 nucleus. The concepts 

 ' ' nucleus ' ' and ' 1 chromatin ' ' differ as do those of ' ' table ' ' 

 and 1 'wood." 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. 8 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; 



