THE NUCLEUS 07 



nature, which has certain definite relations to the life-history and 

 vital activities of the cell. The word " nucleus," on the other hand, 

 as many authorities and more recently Dobell (52) have pointed 

 out, is essentially a morphological conception, as of a body, con- 

 tained in the cell, which exhibits a structure and organization of 

 a certain complexity, and in which the essential constituents, the 

 chromatin-particles, are distributed, lodged, and maintained, in 

 the midst of achromatinic elements which exhibit an organized 

 arrangement, variable in different species, but more or less constant 

 in the corresponding phases of the same species. If this standpoint 

 be accepted, and the nucleus be regarded as an essentially morpho- 

 logical conception, it seems to me remarkable that Dobell, in 

 his valuable memoir on the cytology of the bacteria, should apply 

 the term " nucleus " to a single grain of chroma tin, or to a collection 

 of such grains, and should speak of a nucleus " in the form of 

 chromidia scattered through the cell," or " in the form of a discrete 

 system of granules (chromidia)," phrases which are self-contra- 

 dictory on the principles that he himself has laid down. 



We are confronted, nevertheless, with a considerable difficulty 

 when we attempt to state exactly what amount of organization 

 and structural complexity is essential to the morphological concep- 

 tion of a nucleus. If, as is probable in phylogeny, and certainly 

 occurs frequently in ontogeny (compare Fig. 32), the nucleus arises 

 from a primitive chromidial condition of scattered, unorganized 

 chromatin, at what point does the mass cease to be a chromidium 

 and become a nucleus ? This is a question very difficult to answer 

 at present, a verbal and logical difficulty such as occurs in all cases 

 where a distinction has to be drawn between two things which 

 shade off, the one into the other, by infinite gradations, but which 

 does not, nevertheless, render such distinctions invalid, any more 

 than the gradual transition from spring to summer does away with 

 the distinction between the seasons. Hartmann and his school 

 consider the possession of a centriole as the criterion of a nucleus 

 (see Nagler, 76) ; but it cannot be considered as established, in 

 the present state of knowledge, that all nuclei have centrioles or 

 centrosomes. All that can be said is that, as soon as a mass or a 

 number of particles of chromatin begin to concentrate and separate 

 themselves from the surrounding protoplasm, with formation of 

 distinct nuclear sap and appearance of achromatinic supporting 

 elements, we have the beginning at least of that definite organiza- 

 tion and structural complexity which is the criterion of a nucleus 

 as distinguished from a chromidial mass. 



In the first chapter of this book a distinction was drawn between 

 organisms of the " cellular " grade, with distinct nucleus and 

 cytoplasm, and those of the " bacterial " grade, in which the 



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