32 W. H. GILBURT ON THE STRUCTURE AND 



tion of a clear chemical difference, whatever it may be. Taking 

 advantage of this fact, and without any further reference save for 

 purposes of distinction, it is proposed to call the denser element 

 which eagerly takes the colour " Chromatin," and the one which 

 refuses it " Aehromatin." 



The arrangement of the two nuclear elements is not always alike. 

 Sometimes the chromatin presents the appearance of a distinct re- 

 ticulation or network interpenetrating the whole of the nucleus ; at 

 others it is seen as distinct rods or filaments, while in nuclei which 

 are at rest it sometimes seems as though it was diffused throughout 

 the general substance. Another feature also is sometimes well 

 shown, especially in those nuclei where the chromatin is seen as 

 filaments, and that is the presence of a nuclear-membrane of chro- 

 matin, the rods and filaments running through the enclosed sub- 

 stance, and amongst them the spherical bodies, known as nucleoli, 

 lying free. With regard to the latter, some doubt at present exists, 

 both as to their nature and function ; they take the colour, but not 

 so intensely as the chromatin. Taking now a section, say through 

 the integuments of a young ovule, prepared and stained as already 

 described, and examining it with a power of between five and six 

 hundred, we shall see most of the nuclei presenting an appear- 

 ance somewhat like Fig. 1. This would at first perhaps be some- 

 what misleading, and you might decide at once that the darker 

 parts were granules deeply coloured ; but by careful and slow focus- 

 sing up and down you will soon make out their true character. 

 Were they granules, you would lose some, and others would come 

 into view, but you will find that by watching one you do not lose it, 

 but follow its course, which is generally more or less oblique, prov- 

 ing that it is a rod or filament, and what you first saw as dots or 

 granules were really these filaments of chromatin in optical section. 

 These points are best made out in the large primary nucleus of 

 some embryo-sacs — for instance, of the tulip, from the integu- 

 ments of the ovule of which plant these figures are taken. 

 When a nucleus is about to divide, there is first growth, increase 

 in size, and alteration of form ; whereas it was, whilst at rest, more 

 or less spherical, it now becomes ovoid, its chromatin filaments be- 

 come much coarser, and the appearance is as Fig. 2. The filaments 

 then straighten themselves out and arrange themselves more or less 

 parallel to each other, as Fig. 3, and now contracting towards the 

 centre of the cell as in Fig. 4, they eventually form a plate as in 

 Fig. 5. This plate is the final phase of the first stage, and all that 



