The Structure of the Nucleus. By E. J. Sheppard. 467 



linin-substance. In some cases the entire chromatin-content of the 

 nucleus appears to be condensed into a single mass which simu- 

 lates a nucleolus. 



C. E. Walker, The Essentials of Cytology, 1907, p. 10, line 20. 



When the cell is not in actual process of division, but is in a 

 vegetative condition, the nucleus is surrounded by a membrane 

 which separates it from the cytoplasm. The appearance of the 

 contents of this membrane, the nucleus proper, may vary consider- 

 ably. The most usual appearance, however, is a reticulum or 

 meshwork. In the substance of the threads of this reticulum, or 

 attached to them, are masses of a darkly-staining substance. This 

 threadwork is called linin, and the darkly-staining substance is 

 called chromatin. 



H. A. Haig, The Plant Cell, 1910, p. 101 and elsewhere, speaks 

 of the nuclear membrane and of the network of linin within the 

 nucleus, and the chromatin arranged at intervals thereon. 



In the Eoyal Microscopical Journal, 1912, p. 393, an abstract 

 is given of a paper by H. Stauffacher, in which he states that he 

 does not believe in the existence of a nuclear membrane. 



One of the earliest circumstances which led me to follow up 

 the structure of the nucleus was the observation of the nucleolus 

 being pushed out of its position in the nucleus by the action of 

 the microtome knife in cutting sections of vegetable tissues, as in 

 the growing root -tips of the hyacinth, fritillaria, polyanthus, 

 onion, etc. (see Plate XXI, figs. 3 and 4, and text-fig. 66). The fact 

 that the nucleolus was frequently removed from its usual position, 

 leaving an opening or window in the boundary or peripheral 

 structure of the nucleus, led me to observe that I had here an 

 opportunity, by careful use of the fine-adjustment, of looking 

 through this opening to see the actual condition of the interior. 

 To my astonishment the view presented, instead of showing the 

 reticulated structure described in the extracts quoted, was that 

 of a perfectly homogeneous interior. Similar appearances were 

 obtained, illustrating the same fact in an even more striking manner 

 when examining nuclei of the testis of Triton and salamander, 

 where a portion of the nucleus was cleanly shaved off by the 

 microtome knife (see Plate XXI, figs. 1 and 2, and text-fig. 67). 

 The focusing requires to be very carefully done, otherwise the 

 opposite wall of the nucleus is likely to be brought too readily 

 into focus and mistaken for internal structure. 



Now with regard to the nuclear membrane. The conclusion 

 to be drawn from the extracts already quoted is that the nuclear 



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