﻿1903] BEHAVIOR OF THE CHROMOSOMES 265 



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In a short time, a more regular spirem is formed in which 

 parallel or adjacent portions are frequently connected by fine 

 threads. As I have already shown, the spindle of the second 

 division arises as a multipolar structure {Jig^ 26^. During the 

 formation of the spindle, the chromatin spirem segments into the 

 chromosomes. The segments are collected together within the 

 complex of spindle fibers so that the manner of segmentation 

 can not be determined with certainty. The same would be true 

 if the spirem segments before the nuclear membrane had disap- 

 peared, and it is a matter of little importance whether the spirem 

 segments before or after the disappearance of the nuclear mem- 

 brane. In many cases it seems that the spirem breaks up into 

 pieces equal in length to the two segments of the chromosomes, 

 but this does not always appear to be true [fig. 16'). However, 

 during the transformation of the multipolar complex of fibers 

 into the bipolar spindle, the chromosomes are gradually arranged 

 more regularly into the nuclear plate {figs. 27, 28). Each chro- 

 mosome consists now of two similar 'segments which may or 

 may not be closely applied to each other. Instances have been 

 observed in which the two segments of a chromosome were of a 

 slightly different length. The chromosomes are probably iden- 

 tical with the pairs of granddaughter segments of the first divi- 

 sion, but here, as in Lilium, we do not know definitely whether 

 the two segments of each chromosome are sisters. The chro- 

 mosomes are mostly in the form of rather thick rods more or 

 less curved. They are fastened to the spindle fibers near the 

 ends or at the place of bending. The retreating segments, there- 

 fore, may be nearly straight, hooked, V- or U-shaped. 



Among monocotyledonous plants, apart from the lilies, few 

 perhaps are more suitable to demonstrate the fact of a double 

 longitudinal splitting of the chromosomes during the first mitosis 

 in the pollen mother-cells than Tradescantia virginica. 



Between the time of the segmentation of the chromatin thread 

 into chromosomesandthe development of thespindle, the chromo- 

 somes contract greatly, becoming shorter and thicker. The 

 daughter segments tend to separate except at one or both ends. 

 In case they adhere at both ends, a ring or ring-like chromo- 



