294 THOS. H. MONTGOMERY, 



somes; the end which becomes directed towards the pole of the 

 spindle, which is the end to which the mantle fibres are attached,, 

 will accordingly be termed the central end of the chromosome, and 

 the opposite end will be called the distal end. Since, further, a well- 

 marked polarity of nucleus and cell body soon obtains, to the opposite 

 poles of nucleus and cell body special names shall be given; that 

 pole of the nucleus and cell body which corresponds to the spindle 

 pole of the dyaster stage shall be called the central pole, while 

 the opposite pole of the nucleus and cell body shall be called the 

 distal pole. As we shall see, this polarity of chromosomes, nucleus 

 and cell body can be readily followed unchanged into the prophase 

 of the first maturation division, and is of great importance for determ- É 

 ining the valence and axial relations of the chromosomes in the 

 maturation divisions. 



\Yhen in the dyaster stage the daughter chromosomes have nearly 

 reached the central pole of the cell, their long axes are found to be 

 more or less inclined to the long axis of the spindle (Fig. 48) ; this 

 inclination is most marked in the more peripheral chromosomes of 

 the plate. Hence to view a chromosome from the side one must take 

 neither a strictly pole view nor yet a strictly lateral view of the j 

 spindle, but a view from a point about midway between the two. j 

 Now if we take such an oblique view, and at one pole of the dyaster i 

 pay attention to those peripheral chromosomes whose longitudinal axes I 

 are seen in their entirety, a certain remarkable disposition of them l 

 will be noticed (Figs. 49, 51, 52, and the chromosomes marked x in 

 Fig. 50). Namely, some of the chromosomes may be frequently seen 

 approximated into pairs, in such a way that the central ends of the I 

 chromosomes of a pair lie closer together than do their distal ends. 

 This V-shaped approximation of the chromosomes into pairs is more 

 than a mere coincidence due to the crowding together of the chromo- 

 somes; it is, I think, the first sign of the pairwise union of chromo- 

 somes by which the reduction in their number is effected, and which | 

 results in the formation of bivalent chromosomes. It is, however, not 

 usual at this early stage to find the approximation into pairs, for | 

 usually this movement does not become marked until the chromo- 

 somes have lost their smooth outlines. 



We may at this point try to give a mechanical explanation for 

 this union of chromosomes into pairs, and later take up the observed 

 phenomena of the subsequent stages in succession. In the synapsis 

 stage all the chromosomes become united into pairs, and at the close 



