THE MITOTIC CYCLE 



perhaps with decreasing plausibihty, and in defiance of the principle 

 of Occam's razor. When the spindle is present, direct traction by spindle 

 elements is, in the writer's opinion, the most likely explanation of 

 chromosome movement. However, it must be admitted that in the 

 instances with which this section began, where the nuclear membrane 

 is still intact, it is improbable that this can apply. The fact may be of 

 some significance that the orientations and movements of the chromo- 

 somes both in prophase and in later stages are profoundly influenced 



Figure 46 Diagram illustrating chromosome movements in the monocentric 

 first spermatocyte mitosis in Sciara coprophila a Prophase showing lack of 

 synaptic association of homologues. b Beginning of separation of two groups. 

 The large 'limited chromosomes' which contain only few genes (represented 

 in black) both go toward the pole; the others separate so that the paternal 

 member of each pair of homologues goes away from the pole, and the maternal 

 member toward it. c and d Later stages in the movement of the four paternal 

 chromosomes away from the pole. Note their inverted orientation, with spindle 

 fibre attachment hindmost, e and f The four chromosomes eventually con- 

 verge at a point opposite the single pole and are extruded in a polar-body- 

 like process which becomes separated from the spermatocyte. This later 

 degenerates. From Metz^** {By courtesy, Cytologia). 



by their degree of nucleination. A close study of the movements of 

 heteropycnotic chromosomes in suitable living nuclei might prove of 

 some importance. 



There is some evidence that the chromatin within the intact nuclear 

 membrane can be moved under the influence of electrical forces. Both 

 McGlendon^*^ and Hardy^*^ have shown that this can be effected in 

 the onion root by the application of a direct current and that the nuclear 

 contents migrate towards the anode.* The rough sketches which illustrate 



* The fact that nuclear membrane and chromatin are on the opposite sides of their 

 respective isoelectric points has been mentioned above, on p 52. 



118 



