82 l'U()(Ki:i)IN(iS OK THE 



In short, chromatin cannot be defined solely .by chemico- 

 iniysical tests : it is essentially a biological conception. By 

 chromatin we understand certain grains of substance imbedded 

 in the cytoplasm or aggregated in the nucleus, and playing a 

 definite role in the life-cycle of the organism. In the first place, 

 in reproduction of the simplest type by lission, the chromatin of 

 the daughter-individuals is derived by growth and fission of 

 the chromatin-elemtiits of the parent indi\idual. Secondly, in 

 syngamy (sexual conjugation), tlie constant and essential feature 

 of the process in all its innumerable variations is the union of 

 chromatin from two distinct individuals. Tims chromatin exhibits 

 in itself the primary vital properties of growth, reproduction and 

 individuality — the individuality which is characteristic of living 

 organisms, and which depends primarily on the variability of the 

 living substance. A given granule in a cell cannot be determined 

 with certainty to be chromatin by inspection or by cbemico- 

 l)hysical tests, but only by its relation to the life-cycle of the 

 organism. This is what is meant by saying that the conception 

 of chromatin is a purely biological one. 



Our notion of the living substance infiuences necessarily our 

 ideas as to the primitive form of living organism. It has generally 

 been held that the first living things were relatively large masses 

 of protoplasm consisting of pure cytoplasm, without nuclear 

 elements, which appeared later in evolution. Such hypothetical 

 forms of life were termed Monera by Haeckei, and with a tech- 

 nique less advanced than that of modern times, this distinguished 

 naturalist described organisms which he believed to be true 

 ]\Ionera. But it seems practically certain that no organisms exist, 

 however primitive, which do not contain in some form or another 

 the chromatiu-substance which is the essential constituent of a 

 nucleus. On the view that chromatin represents the primary- 

 living substance, I believe that the first living things were exces- 

 sively minute specks of matter, perhaps even ultra-microscopic. 

 I consider that, of the forms of life existing at the present day, 

 the earliest type is most nearly represented by the minutest 

 Bacteria and allied organisms in which the body is practicdly 

 nothing more than a grain of chrouiatin. The first stages of 

 evolution consisted in a gradual increase in the size of the body, 

 which came to be composed of several or numerous grains of 

 chromatin imbedded in a matrix, the cytoplasm. With further 

 growth in size, the chronuitin, at first scattered through tlie 

 cytoplasm (chromidial condition of the nuclear substance), became 

 aggregated wholly or in part at one spot, and there became 

 organized and combined into a compact body, the nucleus. With 

 the separation of the nucleus and cytoplasm a most important 

 stage of evolution was reached, namely the stage of the cell in the 

 strict sense of the w ord, the starting-point of the evolution of the 

 entire animal and vegetable kingdoms. It would, in my opinion, 

 be of advantage, as conducing to clear thinking, if the term 

 *• cell " could be restricted in its application to that type of 



