262 INTRODUCTION TO CYTOLOGY 



their metaphasic positions with their attachment regions in the equator. ^^ 

 Owing to their compactness and their arrangement in a regular plane, 

 the tetrads are easily counted at this stage if viewed from a polar direction. 

 The anaphasic movement begins very soon. Two chromatids of each 

 tetrad gradually move away from the other two, the movement starting 

 at the attachment regions. As they diverge, it is observed that the 

 chiasmata often appear to offer considerable resistance to the forces of 

 disjunction so that the tetrads may become drawn out into strange 

 shapes which depend upon the relative positions of the attachment 

 regions and the chiasmata as well as upon the number of the latter. 

 Eventually the pairs of chromatids (the dyads) become free and continue 

 their poleward movement. 



Fig. 153. — Plant chromosomes in anaphase /, showing tetrad nature and shapes due 

 to different locations of spindle attachments. {From the works of Allen, Mottier and 

 Strasburger.) 



During the anaphase the two chromatids of each dyad tend to spread 

 away from each other except at the spindle-attachment region, even 

 before the dyads have lost their metaphasic contact (Fig. 153). To what 

 extent this involves an actual division of the matrix or only a separation 

 ■ of two matrices in contact is not altogether clear in many cases. In much 

 of the older literature, particularly that pertaining to plants, it was 

 assumed that this was actually the division of the chromosomes for the 

 second meiotic mitosis; but it is now evident that even in such cases the 

 tetrad chromosomes in the first meiotic metaphase are quadruple with 

 respect to their chromonemata, and that the spreading in the anaphasic 

 dyads represents merely the divergence of two chromatids developed at a 

 much earlier stage. As a result of this spreading of the free ends a dyad 

 with terminal attachment appears as a single V and one with median 

 attachment as a double V. The division of the attachment region is 

 presumably completed in anaphase, telophase, or later. 



After the two diverging groups of dyads reach opposite poles, each 

 group usually begins the organization of a telophase nucleus at once. The 



13 The suggestion has been made that if the attachment region in each synapsing 

 chromosome is the last portion to become effectively double, the two chromatids facing 

 the same pole at this region would be regularly sister chromatids. Whether or not 

 this is actually the case is not known. 



