512 



CELL MECHANICS 



(d) Rate of Movement. The differences found in degree of ter- 

 minalisation in organisms with chromosomes of similar sizes such as 

 Laihyrus and Campanula may be said to be immediately due to 

 differences in the rate of movement relative to external develop- 

 ment. Such differences are found between male and female mother- 

 cells in the neuropteran Macronemurus (Naville and de Beaumont, 

 1933)- They are also found between pollen mother-cells of normal 

 and male-sterile Lathyrus (Faberge and Upcott, unpub.). In the 



/.o /.o 2.1 3.2 5. 



42 



77 



/./ // 2.Z 2.2 2.2 2.2 2.2 



Fig. 144. — First metaphase in normal and male-sterile Lathyrus 

 odoratus, showing correlated difference in terminalisation and 

 spiralisation, cf. Fig. 109, x 4,000 (Upcott, 1936). 



normal the chiasmata reach the intermediate equilibrium position 

 with no fusion of chiasmata. In the male-sterile terminalisation is 

 complete. Here it is possible to show that the abnormality is not 

 due to quicker movement but to slower external development, for 

 the cells do not reach first metaphase until the anther has reached 

 the size at which the normal plant has mature pollen-grains. Corre- 

 lated with the greater movement is a greater spiralisation in both 

 cases (Fig. 144). Similar changes have been produced by 

 abnormal temperatures (Straub, 1936). 



Let us now consider the forces at work. The shapes of bivalent 

 chromosomes at all stages of terminalisation show that a repulsion 

 is acting between all parts of all chromosomes just as it does at other 

 stages of meiosis and mitosis. 



