MORPHOLOGY 



83 



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

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

 surrounding cytoplasm separates itself by means 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 



KII:. S 7. Diagrammatic representation of the different positions occupied by the chromosomes iu 

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

 chromosome ; z, traction-fibres of the spindle. 



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

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

 individuality of the chromosomes is not lost ( 83 ). The young nuclei 

 enlarge, the extranuclear 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 division is described above as it usually takes place iu 

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

 loop-shaped, and the traction-fibres 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 iu the nuclear spindle, and the 

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

 ing 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 U, 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 condition of things results, when the daughter chromosomes separate, which 

 is represented in 4 or 5. The former shows the resulting appearance 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 

 pole. In all cases the separation proceeds from the point of attachment of the 

 traction-fibres. When a paired chromosome is attached to the spindle near one of 

 its ends, the separation of the daughter chromosomes naturally commences near 

 this end ; when the attachment is by the middle of the chromosome the daughter 

 chromosomes remain longer attached by their ends. In Fig. 86 the behaviour of 



