32 



A TEXTBOOK OF THEORETICAL BOTANY 



known as synapsis. This reduces the origiral diploid number to half that 

 number of bivalents. 



One member of each pair has been said above to be maternal in origin 

 and the other paternal. Thus, although the members of a pair are homologous 

 in form they differ in hereditary constitution, each corresponding to that of 

 the parent from which it was derived. 



The chromonemata on coming together, or sometimes at an earlier stage, 

 split lengthways so that the bivalents comprise a tetrad or group of four 

 units called chromatids. The four chromatids, lying side by side, may 

 form unions between those of opposite origin at intervals along their length. 

 These are called chiasmata. 



The matrix now begins to form round the chromonemata, and the 

 resulting bivalent chromosomes contract and thicken until they are much 

 shorter and fatter than any seen in mitosis. At this stage they are scattered 

 throughout the nucleus, an appearance known as diakinesis. We now 

 enter metaphase and the bivalents arrange themselves on the equatorial plate ; 

 the spindle fibres appear and the nuclear membrane disappears. The 



separation that now occurs is not, as in 

 mitosis, the separation of two halves of 

 one chromosome. It is the disjunc- 

 tion, i.e., separation, of two whole 

 chromosomes, the paternal and mater- 

 nal members of the homologous pair 

 which, at synapsis, united to form 

 a bivalent. These now move apart 

 towards the poles of the spindle. 



As disjunction occurs the chias- 

 mata that were formed between the 

 units in the tetrad are broken through, 

 so that an interchange of segments takes 

 place between the chromonemata in- 

 volved. This exchange of paternal 

 and maternal material is known as 

 crossing over (Fig. i6), and it im- 

 plies a redistribution of hereditary 

 material which is of the highest 

 genetical importance. 

 It will be recollected that when the homologous pairs came together 

 the chromonemata split, so that each chromosome which now disjoins con- 

 tains two chromonemata or, with the adherent matrix, two chromatids. As 

 anaphase progresses the matrix on the chromosomes diminishes and the 

 double internal thread becomes visible again. 



When the chromosomes reach the spindle poles they retain their 

 individuality until the second stage, or homotypic division, commences. 

 Two achromatic spindles are formed in the cell at right angles to the first, 

 in such a position that their equators coincide with the poles of the hetero- 



s^ 



I 



I 



I 



B 



Fig. 1 6. — Diagram to illustrate the formation 

 of chiasmata in unsplit homologous 

 chromosomes. (After Sharp.) 



