454 Prof. J. B. Farmer. On the [May 13, 



into the nature of the evidence for the existence of permanent structures 

 within it which will agree with what seems to be demanded by the facts of 

 heredity as elucidated by experiments on breeding. 



The resting nucleus is externally limited by a membrane, within which is 

 contained a gelatinous, more or less reticulated, framework, the linin ; and 

 this is bathed in a more fluid medium, the paralinin. There are also one or 

 more spherical bodies, the nucleoli, which form storehouses of materials that 

 become separated and used up during times of nuclear activity. As long as 

 the nucleus is in the resting condition, it betrays little of that intricate 

 structure which it exhibits at periods of its division. Much of the complexity 

 apparent at those times must be attributed to temporary physical changes 

 proceeding in the colloidal masses which are then undergoing rapid change of 

 state ; but underlying these there is a permanent structural configuration 

 which is constantly preserved and is a determining factor in controlling the 

 course of those cellular changes on which the organisation of the individual 

 ultimately depends. 



When a nucleus is about to enter on division, the linin becomes impreg- 

 nated with a substance termed chromatin, belonging to the nuclein class of 

 compounds. The chromatin, or its precursors, is mainly to be found in the 

 nucleoli as long as the nucleus is at rest ; it is only during the period of 

 division that it becomes aggregated in the linin. But the chromatin is not 

 evenly distributed through the linin. It appears as very regularly arranged 

 discontinuous swellings in the thread. These small swellings, formed of 

 chromatin and linin, have been called chromomeres. The linin, made up of 

 alternate discs of chromatin- and non-chromatin-oontaining segments, now 

 either forms a convoluted filament, which finally breaks up into a definite 

 number of segments, the cJuvmosomes ; or else these bodies are visible at the 

 first onset of division as discontinuous areas of linin mapped out by the 

 accumulation of chromatin in groups of chromomeres. The chromosomes are 

 commonly stated to be constant in number for a given organism, but this is 

 certainly not universally true, especially in the vegetative, or somatic cells. 

 On the other hand, in the reproductive cells the number is on the whole very 

 uniform, and even peculiarities of form distinguishing certain chromosomes 

 may continually recur. 



The chromosomes then take up definite positions at the equator of the 

 nucleus, and each chromomere in the thread has already split into two exactly 

 similar halves. This carries with it the longitudinal fission of each chromo- 

 some as a whole, and the two halves then move apart and are finally 

 distributed, one to each of the two daughter nuclei which are thus formed. 



The chromosomes are the only parts of the original nucleus of which it 



