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THE AMERICAN NATURALIST [Vol. L 



of animals and plants. Thus in the first place a distinct 

 nuclear membrane may be entirely absent and the chro- 

 matin-elements, whether occurring in the form of a com- 

 pact karyosome or of a clump of grains, are lodged simply 

 in a vacuole in the cytoplasm, that is to say in a cavity 

 containing a watery fluid of nuclear sap in which the 

 mass or masses of chromatin are suspended. It is a moot 

 point, to which I shall return again, whether in nuclei 

 of this simple type the linin-framework may sometimes 

 be absent altogether, or whether it is invariably present 

 in at least a rudimentary form, appearing as delicate 

 threads (in optical section) extending from the chromatin- 

 masses to the limiting wall of the nuclear vacuole, or be- 

 tween the grains of chromatin themselves. When such a 

 framework can be detected, the nucleus acquires the ap- 

 pearance, in preserved preparations at least, of possess- 

 ing a definite structure and is often termed a resting 

 nucleus; many observations have shown, however, that 

 the nucleus during life is undergoing continual internal 

 movements and re-arrangements of its parts and is by 

 no means at rest. The linin-framework can not, there- 

 fore, be regarded in any way as a rigid skeleton, but must 

 be interpreted as an alveolar framework similar to that 

 of the general protoplasm and equally liable to move- 

 ment, displacement and change. 



From this survey, necessarily most brief and super- 

 ficial, of the manner in which the nuclei of Protists may 

 vary from the type of nucleus described in the text-books, 

 it is at once evident that the essential part of the nucleus 

 is the chromatin, and that the other structural constitu- 

 ents of the nucleus, namely, membrane, framework, and 

 plastin or nucleolar bodies, are to be regarded as acces- 

 sory components built up round, or added to, the primary 

 nuclear material, the chromatin. Even with regard to 

 the nuclei of Metazoa it is maintained by Vejdovsky that 

 at each cell-generation the entire nucleus of the daughter- 

 cell is produced from the chromosomes alone of the 



