iiS MEIOSIS IN DIPLOIDS AND POLYPLOIDS 



The observation of the structure of anaphase chromosomes is of 

 great importance. It shows, first, that what might have been 

 regarded at metaphase as " points of contact " between the 

 chromosomes really were chiasmata. This is particularly significant 

 in organisms (such as Pisum or Qinothera) where it has not been 

 possible to trace the development of the chiasmata in prophase. 

 It shows, secondly, the relationship of the exchange undergone at 

 one chiasma to that undergone at the next. Two types of relation- 

 ship are to be expected (Newton and D., 1930). In one the second 

 chiasma restores the association of chromatids that was lost at the 

 first chiasma ; the two chiasmata may then be said to be compensa- 

 ting or comparate in a purely observational sense. In the other 

 type the second chiasma does not restore the original association. 

 The chiasmata are non-compensating or disparate. Both these 

 types of relationship are found (Fig. 36) where anaphase chromo- 

 somes have been studied and are readily distinguished. They are 

 derived from different kinds of relationship between the crossing- 

 over at the two chiasmata which will be considered later (Ch. VII). 



(ix) Interphase. The chromosomes pass to the pole in half the 

 number characteristic of somatic mitoses. But since each consists 

 of two chromatids instead of one, these are present in the normal 

 number. We may say, therefore, the number of centromeres is 

 reduced, the number of chromatids is unreduced. 



When the chromosomes reach the pole they either reconstitute 

 daughter-nuclei or they pass directly into a second division. In 

 the first case the daughter-nuclei may pass into a fairly complete 

 resting stage, with uncoiling of relic spirals and development of 

 nucleoli. In these circumstances the chromosomes regain, or 

 partly regain, their normal mitotic length. Otherwise, as in many 

 animals and dicotyledons, they remain contracted to the same 

 degree as at the first division. 



The loss of contraction is shown to be due to the occurrence of 

 an interphase by an abnormahty in Gasteria. Under exceptional 

 conditions, doubtless both genetic and environmental, shortening 

 occurs in the length of the interphase with a corresponding lack of 

 recovery to the length of a somatic mitosis found in the normal 

 nuclei (Tuan, 1931 ; D., 1936 d). 



