16 



THE AMERICAN NATURALIST 



[Vol. L 



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 homologue or ana- 

 logue 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. Conse- 

 quently, in studying the functions of the nucleus the 

 human cytologist finds himself in the same difficult posi- 

 tion that an intelligent 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 proper- 

 ties of the essential nuclear substance, the so-called chro- 

 matin. To define, or characterize adequately, this sub- 

 stance is a difficult task. The name chromatin is derived 

 from the fact that this substance has a peculiar affinity 

 for certain dyes or stains, so that when a cell is treated 

 with the appropriate coloring reagents — with so-called 

 nuclear stains — the chromatin in the nucleus stands out 

 sharply, by reason of being colored in a different manner 

 from the rest of the cell. In consequence, the statement 

 is frequently made, in a loose manner and without reflec- 

 tion, that chromatin is recognized by its staining reac- 

 tions, 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 is recog- 

 nized 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 



