SOMATIC MITOSIS AND CHROMOSOME INDIVIDUALITY 1 17 



Detailed Description of the Behavior of the Chromosomes in Somatic 

 Mitosis. 1 — In the present account we shall depart from the order usually 

 followed in descriptions of mitosis. Instead of commencing with the 

 resting nucleus and tracing the steps leading to the formal ion of two 

 daughter resting nuclei, we shall begin the description with the fully 

 formed chromosomes as they appear at the metaphase and follow them 

 through anaphase, telophase, resting stage, and prophase to the nexl 

 metaphase, when they are again clearly seen. This is done in order thai 

 the account of the telophasic transformation of the chromosomes to 

 form a resting reticulum, and the prophasic condensation of the Latter 

 to form chromosomes, may be given without interruption, which -('(•in- 

 advisable in view of the nature of certain questions which are later to 

 be discussed in the light of chromosome behavior. 



Metaphase (Fig. 50, A). — As the chromosomes arrange themselves 

 upon the spindle preparatory to their anaphasic separation their double 

 nature is clearly evident. The two halves may lie very close together 

 and in the case of long chromosomes may be somewhat twisted about 

 each other. When they lie a little apart they may often show small 

 connecting strands or anastomoses; in the immediately preceding stages 

 (late prophase) the halves are usually pressed tightly together, so thai 

 these anastomoses appear to be due to mutual coherence at certain 

 points when the halves move slightly apart after the disappearance of 

 the nuclear membrane. As the double chromosomes take their places 

 on the spindle, the spindle fibers become attached to them, not to all parts 

 but to a particular portion of each. In the case of long chromosomes 

 the point of attachment is often at about the middle, whereas in shorter 

 ones it is commonly near one end. At their points of attachmenl to 

 the spindle the double chromosomes all lie with their halves superposed 

 (one half toward each spindle pole) and in a single plane; those portions 

 to which no fibers are attached may extend in various directions with do 

 regular arrangement. 



Anaphase (Fig. 50, B-D).- -The daughter chromosomes (the hah 

 of the double chromosomes seen at metaphase) now begin to separate, 

 first at the point of insertion, and gradually move away from the equa- 

 torial plane. Owing to the different locations of the points of fiber attach- 

 ment, and also to the fact that the free ends of the chromosomes occupy 

 various positions, the chromosomes, unless they are wry short, may new 



1 This description is based on the author's accounts of somatic mitosis in Vicia 

 faba (1913) and Tradescaniia virginiana (1920). In these papers, especially in the 

 first, there is presented a more extensive comparison of the results of other investi- 

 gators than can he given here. Comparative studies have shewn that in general 

 the present description is widely applicable to mitotic phenomena in plants and ani- 

 mals, although many modifications in detail arc known, particularly in forms with 

 small chromosomes. A useful list of works on mitosis in angiosperms is given by 

 Picard (1913). 



