504 



CELL MECHANICS 



The configurations observed depend for their form, as we have 

 seen, on the positions and numbers of their chiasmata. The forms 

 of the chiasmata themselves depend on their positions. They 

 may be classified in the first place according to whether they are 

 interstitial or terminal. An interstitial chiasma is always of one 

 kind. The only conceivable complication of this form is that 

 where two such chiasmata involving the six chromatids of three 

 chromosomes might coincide. This has not been observed. The 

 terminal chiasma, on the other hand, has various complications. 

 Several such chiasmata may coincide, and we then. have a multiple 

 chiasma. Again a chiasma which is terminal for one chromosome 



A B 



Fig. 140. — Diagram of chromatids showing different types of chiasma 

 observed at metaphase and early stages. A, interstitial chiasma. 

 B, terminal chiasma. C, triple (terminal) chiasma. D, quad- 

 ruple chiasma. E, imperfect terminal chiasma. F, imperfect 

 lateral chiasma (asymmetrical type — for symmetrical cf. 

 Fig. 86, Ci). G, lateral-terminal chiasma, one chromosome 

 having a reduplicated segment. (From D., 1931 c.) 



may be interstitial for its partner, with a part of which it is 

 homologous, and then we have a lateral chiasma. Lateral chiasmata 

 are of two kinds : those which must be supposed to arise from 

 terminalisation of a single chiasma (i.e., a single cross-over on the 

 chiasmatype hypothesis) or several non-compensating chiasmata, 

 and those which must be supposed to arise from two reciprocal 

 chiasmata. The first gives a symmetrical configuration, the second 

 an asymmetrical one. Multiple combinations of chiasmata, arising 

 presumably through pairing of reduplicated segments in the same 

 chromosome, may include lateral chiasmata. 



When terminalisation is complete, every chromosome end that 

 is associated by a chiasma with another end remains so associated 

 through diakinesis and metaphase until anaphase, so that the same 



