THE president's ADDRESS. ITS' 



possible transition from the one to the other. The difference- 

 between these two types depends on the condition under which 

 that pecuHar substance occurs for which we may use, in quite 

 a general sense, the term chromatin. In every cell of an 

 animal or plant, and probably in every Protist organism, there- 

 is found a certain amount of substance remarkable for its affinity 

 for certain colouring matters, and still more remarkable for the- 

 part it plays in all vital processes. From the first of these 

 characteristics the name chromatin has been given to it. It is 

 a substance, or combination of substances, of a very high degree 

 of chemical complexity, perhaps more complex than any other 

 substance, but it is by no means of uniform chemical com- 

 position ; on the contrary, the chromatin in any given sample 

 of living matter probably differs chemically, to a greater or less 

 degree, from that found in any other sample. The term 

 chromatin implies, in short, a biological or physiological, but 

 not a chemical unity. It is a substance generally easy to 

 recognise, but extremely difficult perhaps impossible at present 

 to define. 



In the lower type of Protist organisation, which is exemplified 

 by the ordinary Bacteria, and which I shall therefore speak of 

 shortly as the bacterial grade, the chromatin is present in the 

 form of scattered granules, now generally. termed " chromidia." 

 In many cases the whole body in this type appears to consist of 

 little, if anything, more than a single minute speck or thread 

 of chromatin ; in other cases several distinct grains can be made 

 out, some of them possibly not true chromatin, but substances 

 out of which chromatin is built up, or into which it breaks down, 

 in the body. 



In the second type of organisation, a certain amount of the- 

 chromatin may still be present in the scattered chromidial 

 condition, but the greater part, and in most cases all, of 

 the chromatin is aggregated into a compact mass termed 

 the 7iudeios. Apart from the nucleus the remainder of the^ 

 protoplasmic body is made vip of a distinct zone or region- 

 called the cytoplasm, scarcely recognisable, if at all, in the- 

 bacterial type of organisation. The nucleus shows a wide 

 range of structural difierentiation and ever-increasing com- 

 plexity of organisation. Consisting in the simplest cases of 

 perhaps nothing more than a compact lump of chromatin, in. 



