Lecture XXVIII. 251 



nucleus so that the nucleus as a whole presents a granular 

 appearance. Distinct from these and apparently floating free 

 in the nuclear fluid are one or more somewhat larger granules, 

 also of high refringency. These are called the nucleoli and are 

 often salient objects in the resting nucleus. When such a resting 

 nucleus is killed and stained by suitable reagents, it is found 

 that the granules on the nuclear thread fix a basic stain readily. 

 They probably react in this manner owing to the fact that they 

 contain a large proportion of nucleinic acid, which is a complex 

 organic acid possessing a high percentage of phosphorus. The 

 substance forming these granules, owing to its readiness to fix 

 stains, is called chromatin. The material forming the viscid 

 thread itself does not take basic stains and is called linin. The 

 nucleoli are also stainable, but with acid stains ; and hence the 

 chromatin granules and the nucleoli may be made to stand out 

 plainly in preparations stained with a mixture of an acid and a 

 basic stain. The nuclear fluid, on the other hand, does not take 

 stain. The nuclear membrane fixes basic stains, but it is so 

 thin that its coloration is feeble. The different behaviour of 

 these parts towards stains has been most helpful in studying 

 the structure of the nucleus; and with proper controls gives some 

 indication of the chemical nature of its constituents. 



When a nucleus is about to divide the bulk of the chromatin- 

 granules increases, and the anastomoses of the nuclear thread 

 are gradually dissolved. The chromatin-granules become uni- 

 form in size and space themselves uniformly along the thread. 

 In this state they are often called chromomeres. The thread 

 now breaks up into a number of short lengths the chromosomes. 

 The nucleoli disappear and possibly add their substance to the 

 chromosomes. The chromosomes arrange themselves in the 

 equatorial plane of the nucleus, and form a disc usually called 

 the equatorial plate. The number of chromosomes is usually 

 under 100. The number is constant in the vegetative tissues 

 of each plant. About this time the nuclear membrane breaks 

 down, and some of the finely granular, almost homogeneous 

 surrounding cytoplasm invades the nuclear cavity. In living 

 cells this mass of cytoplasm appears as a highly refringent barrel- 

 shaped structure, lying in the space previously occupied by the 

 nuclear fluid, with the chromosomes, each now bent into an 

 L-shaped form, lying round its middle. The blunt ends of this 

 mass terminate at the poles of the nucleus. When the cell is 

 killed and stained this structure appears composed of a sheaf 

 of fibres, and is called, owing to the difficulty of staining it with 



