12 THE AMERICAN NATURALIST [Vol. L 



the nucleus, which undoubtedly possesses an importance in 

 the life and functions of the cell far greater than would 

 be inferred from the name given to it. A nucleus, as 

 seen in its typical form, has a limiting membrane enclos- 

 ing a framework composed of a substance termed "linin." 

 The framework has the form of a network, which is prob- 

 ably to be interpreted, primitively at least, as the optical 

 expression of an alveolar structure similar to that seen 

 also in the cytoplasm, but of coarser texture, and the ap- 

 parent "threads" of the linin-framework may then be 

 the optical sections of the partitions between neighboring 

 alveoli. Such an interpretation does not exclude the pos- 

 sibility of the formation of real threads or fibers in the 

 framework in certain cases or during particular periods 

 of nuclear activity; just as fibrous structures may arise 

 in the alveolar cytoplasm also. The cavities of the frame- 

 work contain a watery fluid or nuclear sap, probably of 

 the same nature as the fluid enchylema or cell-sap con- 

 tained in the alveolar framework of the cytoplasm. At 

 the nodes of the alveolar framework are lodged grains or 

 masses of chromatin, a substance which must engage our 

 most particular attention, since it is the essential constit- 

 uent of the nucleus, universally present in all nuclei, 

 whether of the simplest or of the most complex types. 

 In addition to the chromatin-grains, which are distributed 

 in various ways over the linin-framework, there are to be 

 found usually one or more masses termed nucleoli, com- 

 posed of a material which differs from chromatin in its 

 reactions and has been termed plastin. 



In the foregoing paragraph I have described in general 

 terms the typical nucleus of the text-books, as found com- 

 monly in the cells that build up the bodies of ordinary 

 animals and plants. The minutiae of the details of struc- 

 ture and arrangement of the constituent parts may vary 

 infinitely, but the type remains fairly constant. When 

 we come, however, to the nuclei of the Protista, such pro- 

 nounced modifications and variations of the type are met 

 with that a description in general terms is no longer pos- 



