i8 CYTOLOGY 



CHAP. 



(2) Chromatin and Linin 



Chromatin is distinguished from Hnin by its much greater affinity for 

 most stains. It is from this feature that it gets its name, while the hnin 

 is often called, in contradistinction, achromatin. By many cytologists 

 chromatin is believed to be composed of very minute granules, or 

 chromioles (see Heidenhain, 1911). The blocks of chromatin* seen in 

 most resting nuclei, or the chromomeres of prophase chromosomes, are 

 aggregations of numbers of chromioles. 



In many nuclei the meshes of the chromatin and linin spongework are 

 filled with a granular mass, as if the karyolymph had been precipitated 

 by the fixative. According to Heidenhain (see 191 1), however, the 

 granules are granules or chromioles of a substance allied to the true 

 chromatin, and known as oxy chromatin, the chromatin proper then being 

 designated basichromatin. This terminology is based on the fact that 

 the chromatin in the usual sense of the term, i.e. the basichromatin, has 

 a special affinity for the basic aniline dyes, while the oxychromatin stains 

 more readily with acid dyes. (These terms do not refer to the acid or 

 alkaUne reaction of the solutions of the stains, but to their chemical 

 derivation.) Unless the contrary be stated, the word chromatin as used 

 in this book refers to the basichromatin. 



The interpretation of the granular mass in the meshes of the true 

 chromatin spongework as composed of pre-existing granules and not 

 due to precipitation of the karyolymph by the fixative is made more 

 probable by the observations of Gross (1916), who was able to observe 

 these granules in the living nucleus. 



There is some reason to believe that the two kinds of chromatin (if 

 indeed the oxychromatin be entitled to this designation) are different 

 phases of the same substance, for they appear to be convertible into each 

 other — as, for instance, in the growth stage of the oocyte (cf . Jorgensen, 

 19 13). Moreover, in the prophase of all mitoses the oxychromatin 

 disappears completely, either by solution, or by conversion into or absorp- 

 tion by the basichromatin. Hence a nucleus rich in oxychromatin 

 presents a characteristically different appearance in the resting and 

 prophase stages ; in the latter the spireme filaments stand out sharply 

 in a perfectly clear karyolymph, while in the former the basichromatin 

 spongework is partially obscured by the mass of faintly stained oxy- . 

 chromatin. In the young daughter nuclei the oxychromatin is formed 

 anew, probably at the expense of the basichromatin. Doubtless in 

 correlation with its disappearance in mitosis, oxychromatin is generally 

 very scanty, or altogether absent, in nuclei which are undergoing rapid 

 multipHcation. 



It is generally supposed that the chromatin (in the form of chromioles) 



