THE NATURE OF CHROMATIN 19 



division in the chromosomes seems to ensure that half of 

 every individual portion of every individual chromosome 

 will be handed on throughout the succeeding generations of 

 cells, for the division of the chromosome does not appear to 

 be merely a non-selective division of the bulk of substance 

 forming it. 



Unfortunately for the theory which identifies the chro- 

 matin with the idioplasm of Nageli, several facts have been 

 observed recently which seem to show that the chromatin is 

 not the permanent substance that it was formerly believed 

 to be. During certain stages preparatory to the divisions 

 which precede fertilisation the amount of chromatin con- 

 tained in the spireme, waxes and wanes in the most remark- 

 able manner, in the case of several animals at any rate. To 

 such an extent does this happen, that in some cases the 

 spireme seems to consist almost entirely of linin, containing 

 little or no chromatin within it. 1 This seems to suggest that 

 the chromatin is rather a secretion or excretion of the linin 

 than a permanent constituent of the cell. In our search for 

 a permanent constituent in the cell, which divides in a 

 selective manner, we are thus thrown back on the linin. As 

 has already been stated, the chromatin is surrounded by an 

 envelope of linin. The spireme sometimes consists of a 

 tube, sometimes of a ribbon or thread of linin, in which are 

 contained granules of chromatin. The framework of the 

 chromosomes is also formed of linin containing chromatin, 

 so that were the chromatin washed out of the spireme and 

 the chromosomes, we should still have the same coiled thread 

 or ribbon, and the same U or V shaped bodies formed entirely 

 of linin. If, therefore, there is a permanent constituent of 

 the cells which is handed on individually from generation to 

 generation, it would appear to be rather the linin than the 

 chromatin. What has been said with regard to the selective 

 mode of division which ensures that an exact representative 

 half of each chromosome is handed on at each division, 

 applies to the linin even more forcibly than to the chromatin. 



w 



1 Walker, C. E., Essentials of Cytology, Constable, London, 1907. 



