SECT. I 



MORPHOLOGY 



83 



the arrangement of the chromosomes in the equatorial plane (5). After 

 the chromosomes have split longitudinally into the daughter chromo- 

 somes, the latter are separated and drawn towards the two poles by 

 the contraction of the traction-fibres (8, 9). The supporting-fibres 

 afford the necessary resistance in the process. The spindle fibres can 

 often be traced to the limiting layer of the cytoplasm, and their 

 attachment to this determined. In forming the daughter nuclei, the 

 free ends of the chromosomes first become drawn in (10), and the 

 surrounding cyptoplasm separates itself by meaiis of a protoplasmic 

 membrane, the nuclear membrane (11), from the developing nuclei. 

 Within the nuclear cavities which are thus produced the chromo- 

 somes again assume a reticulate structure and unite with one another 

 to form a network within which their individual limits are not dis- 

 tinguishable. We are compelled, however, to assume that the 



Fio. 87. — Diagrammatic represeutation of the different positions occupied by the chromosomes on 

 the spindle and during their separation, a and b, daughter chromosomes of one mother 

 chromosome ; z traction-fibres of the spindle. 



individuality of the chromosomes is not lost. The young nuclei 

 enlarge, the extra - nuclear nucleoli disappear in the surrounding 

 cytoplasm, and one or more nucleoli at length appear in the nuclei ; 

 finally the resting condition is again attained. 



The process of nuclear divisiou is described above as it usually takes place in 

 the young tissues of more highly organised plants. The chromosomes are usually 

 loop-shaped, and the traction-iibres become attached to the middle point of the 

 loop, less commonly toward an end. The traction-fibres from the two poles attach 

 themselves respectively to the two daughter chromosomes of each pair. The 

 orientation of the pairs of daughter chromosomes on the nuclear spindle, and the 

 way in which they separate from one another, are represented in the accompanying 

 diagram (Fig. 87). In (1) the two shanks of each pair of chromosomes lie nearly 

 in the equatorial plane, and each of the daughter chromosomes on their separation 

 (2) assumes the form of the letter (j, remaining attached by the shanks. More 

 commonly, while one of the shanks of the paired chromosome lies in the equatorial 

 plane, the other is directed towards one of the two poles (3). In this case a con- 

 dition of things results, when the daughter chromosomes separate, which is 

 represented in 4 or 5. The former shows the resulting ap^iearance when the 

 daughter chromosomes remain for a time attached to one another at both ends ; 

 the latter when separation soon follows at the end directed towards the nuclear 



