STRUCTURE OF THE NUCLEUS. 



181 



directions, which interlace and join with one another throughout the nucleus to form 

 the reticulum which is usually observed (fig. 209, c). When, however, a nucleus is 

 about to undergo division by karyokinesis these secondary twigs become again with- 



nrimfcrtf 



Fig. 209. DIAGRAMS TO SHOW THE GENERAL ARRANGEMENT OF THE CHIEF CHROMATIC FILAMENTS ix 



A NUCLEUS. (Rabl.) 



A, nucleus viewed at the "pole" end; 7, Z, looped extremities of the chromatic filaments; I, I', 

 irregular filaments. 



B, nucleus viewed at the " anti-pole " end. The free ends of the chromatin loops are seen inter- 

 digitating. 



C, nucleus viewed from the side, the " pole " end being uppermost. On the left side only the pri- 

 mary filaments are shown ; on the right side the secondary filaments, which produce the network of 

 the resting nucleus, are represented. 



drawn into the parent loops of chromoplasm, and the typical arrangement of the 

 loops becomes manifest. 



The nuclear filaments are not structureless, although it is difficult to make out 

 any structure in them when they form a close reticulum. But when they form a 

 convolution or skein within the nucleus they are often sufficiently large 

 and evident to permit a definite structure to be made out. Indications of 

 such structure were first described by Balbiani in salivary cells of an insect-larva 

 (Chironomus), in which the convoluted nuclear filaments show well-marked trans- 

 verse striations as if made up of alternate dark and light bands. When this appear- 

 ance is more closely investigated it is found to be due to the existence of minute, 

 highly-refracting particles, which are imbedded in regular series in a clear homo- 

 geneous matrix (fig. 210). These particles have a strong affinity for colouring matters, 

 and it is in virtue of their presence that the nuclear filaments become deeply 

 stained by hsematoxylin, safranin, and other substances ; hence the material of which 

 they are composed is known as chromatin, or chromatic substance, and the re- 

 mainder of the nucleus is termed in contradistinction achromatin, or achromatic 



