62 



FUNDAMENTALS OF CYTOLOGY 



normally constant in number in a given kind of plant, quickly become 

 arranged in such a way that a certain specialized portion of each of them 

 occupies a position in the equatorial plane. This portion consists of the 

 spindle-attachment regions, or kinetochores, of the two chromatids. The 

 two kinetochores face opposite spindle poles, while other portions of 

 the chromosome maj^ lie in any position. When this stage is reached, the 

 nucleus is in the metaphase of division. 



As the kinetochores take up their positions at the equator, a new ele- 

 ment appears in the mitotic figure. At the kinetochore of every chromatid 

 there appears a small mass of material which gradually extends poleward 

 through the spindle substance as a so-called tractile Jibe?-. Whether this 

 represents a local modification of the spindle substance, a fluid extruded 



i^K^Kl.-- -1^1^^ '<:?^Jv\ ''irr 'u^ 







1 2 3 4 



Fig. 42. — Spindle development in root tip of hyacinth. Explanation in text. {After 



W. Robyns.) 



from the chromatid, or an actual pseudopodium-like extension of the 

 chromatid is not yet agreed upon b,y cytologists. The fact that it some- 

 times contains a Feulgen-positive material strongly suggests its chromo- 

 somal origin. That it actually exerts a tractile force is seriously doubted. 

 In the anaphase the two chromatids of each chromosome separate and 

 pass toward opposite poles, the kinetochores moving ahead along the 

 course of the tractile fibers. After each chromatid becomes free from the 

 other and goes its independent way, it should be referred to as a chromo- 

 some, the two half-chromatids being advanced accordingly to the rank 

 of chromatid. In the anaphase, as in the metaphase, the general mor- 

 phology of the chomosomes is usually well displayed, for they tend to 

 lie well separated from one another and show the location of their kine- 

 tochores clearly (Fig. 43). Long chromosomes may present a very 

 confusing appearance during the earlier portion of the anaphase, for 

 even though the kinetochores pass poleward regularly, the other portions 



