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1903] BEHAVIOR OF THE CHROMOSOMES 277 



It is generally assumed that the two granddaughter seg- 

 ments of each chromosome appearing in the nuclear plate of the 

 second mitosis are sisters and identical with the j^airs of seg- 

 ments of the anaphase of the first division. This can only be 

 true when the splrem segments in exactly the same manner in 

 which it was constructed, assuming that the pieces separated by 

 the breaking up of the spirem consist of the two segments of 

 each chromosome. But on the other hand, if the spirem break 

 up so that the individual segments are separated from the start, 

 then sister segments may be brought together in pairs in the 

 nuclear plate. It is evident in many instances that the pieces 

 into which the spirem falls does consist of the two segments of 

 each chromosome in the second division, and that these are 

 brought side by side by the folding of the piece or by the two 

 parallel members of a loop or turn approaching each other. 



However, as has been pointed out in a preceding paragraph, 

 should the spirem segment in a different manner with each piece 

 representing two segments, the two segments of the chromo- 

 somes would not necessarily be sisters. In such cases, the pos- 

 sibility is not excluded that two or more sister chromosomes 

 may fall to the same nucleus. Moreover, as the identity of the 

 individual chromosomes is lost in the daughter nucleus, it is not 

 possible to demonstrate objectively whether the segments are 

 sisters or not. 



There is also no basis in fact which will justify the doctrine 

 that one chromosome is qualitatively different from another. 



The development of the spindle seems to show that the chromo- 

 somes are oriented in the nuclear plate by a pushing and pulling 

 action of the spindle fibers. There is some evidence to indicate also 

 that during the anaphase a pushing is exerted upon the retreat- 

 ing segments, yet this view can not be accepted without the 

 greatest reserve, as the presence of a large bundle of fibers 

 extending from each daughter segment to the poles argues 

 strongly in favor of a pulling or contracting of the fibers. 



According to the best cytological evidence the micro- and 

 macrospore mother-cells are homologous. That type of devel- 

 opment of the embryo-sac in which four potential macrospores 



