48 



THE CELL 



Of*" 



consist of paranuclear, and on this account Flemming has called 

 them net-knots, in order to distinguish them from nucleoli. 



The framework of the nuclei of the various animal tissue cells 

 may be fine or coarse. In the latter case it consists of only a 

 few strands, so that " it hardly deserves the name of a net or 

 framework." As a rule, the nuclei of young, embryonic and 

 growing tissues possess, as Flemming has observed, networks 

 coarser than those of similar tissues in the adult. 



For the most part the nuclear framework is composed of two different 

 substances, linin and nuclein; of these the latter alone is capable of 

 absorbing and retaining the ordinary staining reagents. The two 

 substances are generally so arranged that the nuclein, in the 

 form of coarser and finer granules, is evenly distributed upon and 

 throughout the colourless linin. When the meshes of the frame- 

 work are very fine (Fig. 24) it may be very 

 difficult, or indeed impossible, to distinguish 

 the two substances from one another. In a 

 coarser network, such as is represented in 

 Fig. 25, it is much easier to do so ; here a 

 resting nucleus from the protoplasmic lining of 

 the wall of the embryo-sac of Fritillaria im- 

 perialis is portrayed. According to Stras- 

 burger's description, the delicate framework 

 threads as a rule do not become stained ; hence 

 they must consist of linin. Coloured nuclein 

 granules of varying size are seen to be de- 

 posited upon them. In addition a number of 

 variously sized nucleoli are to be seen. 

 If any one should wish to convince himself of the fact that a 

 special framework of linin is present in the nucleus, he cannot do 

 better than examine the nuclei of the sperm-mother-cells, of the 

 round worm of the horse (Fig. 26). Daring the early stages of 

 division, all the nuclein is gathered into eight bent hook-shaped 

 rods, which collect together into two bundles; they are, as it were, 

 suspended in the nuclear cavity, for colourless threads of linin con- 

 nect them both to the nuclear membrane and to one another. It 

 is impossible for these threads to be coagula in the nuclear sap, 

 produced by the use of reagents, since they are invariably regu- 

 larly arranged. Similarly their chemical reaction and their be- 

 haviour during the process of division show that they are composed 

 of a substance which differs somewhat from nuclein and para- 

 nuclein. 



-8QflF< 



Fig. 25. Fritillaria 

 imperialis. A resting 

 nucleus (after Stras- 

 burger, Fig. 191 A). 



