224 FINE-STRUCTURE OF PROTOPLASM II 



objected that in this case not only the fibrils connected with the chro- 

 mosomes must be shortened but also those seemingly stretching from 

 pole to pole. Schmidt (1939a) has shown, however, that the double 

 refraction of the spindle fibrils is extinguished at the equator. In other 

 words, the fibres running from pole to pole appear to be interrupted. 

 The intermediate body formed at the equator (phragmoplast) is iso- 

 tropic. When the chromosomes are drawn apart from each other, the 

 birefringence of the fibrils decreases, as is to be expected in the con- 

 traction of protein fibres. 



The strain theory cannot explain why the chromosomes migrate to 

 the equator in the metaphase. To imagine a mechanism which ac- 

 counts for that movement, rather complicated assumptions must be 

 made. Ostergreen (1950) thinks the attracting forces of the poles 

 increase with increasing distance. Therefore, the centromere (kineto- 

 chore) of the metaphase chromosome is only in equilibrium when 

 located midway between the two poles. After the cleavage of the 

 metaphase chromosome, an additional hypothesis is needed to explain 

 anaphase, viz. that the kinetochores of the two daughter chromosomes 

 have a polar character, so that only their side turned towards one of 

 the poles is attracted. 



It is obvious that a hypothesis resting on such uncertain grounds 

 is no better than the assumption that the chromosomes have active 

 locomotion at their disposal which allows them to move to and fro. 



c. Fine-Structure of the Chromosoiues 



The chromosomes differentiate from the nuclear reticulum in which 

 they are preformed. During the prophase of cell division they disen- 

 tangle, shorten and become independent. The membrane of the 

 nucleus which is the semi-solidified peripheral part of the nuclear sap 

 disappears. After the destruction of this boundary, the karyolymph 

 readily mixes with the cytoplasm. 



The chromosomes often possess two arms. The connecting part 

 between these arms is somewhat constricted (primary constriction) 

 and cannot be stained by Feulgen's reagent; it is anucleal. The con- 

 striction serves as a point of contact for the spindle fibre. This region 

 often possesses a clearly visible boundary and is then designated as 

 centromere or kinetophore. In addition to the primary constriction, 

 so-called secondary constrictions are sometimes found, where the 



