The individuality of the chromosomes and their serial arrangement, etc. 123 



phases of the somatic division, and yet with some striking similarities. 

 In both there is a marked early growth of the nucleus. In somatic pro- 

 phases the chromosomes maintain a rather constant form. In preparation 

 for synapsis, however, the chromosomes change their shape and the 

 chromatin is drawn out into a delicate thread of considerable length, 

 a condition to which there is nothing comparable in the somatic divisions 

 of the plant and which is plainly an entirely new condition of the chro- 

 matin material, a point which is discussed further on. 



The synaptic stage with the changes immediately preceding and 

 following it, evidently lasts for some time, perhaps several days. As 

 the knot loosens up we find a thick continuous spirem which appears as 

 one solid homogeneous thread. The loosening continues until the thick 

 spirem (pachyneme) is distributed quite evenly in the nuclear cavity 

 where it is variously twisted about as is shown in figure 23. The stages 

 of this loosening can be followed in the greatest detail, but there is no 

 visible evidence regarding the mechanism of the process. At this stage 

 no longitudinal split can be discerned in the thick spirem and through- 

 out its entire length there appears no indication of the limits of the 

 individual chromosomes. 



I have not found intermediate stages between the spirem just de- 

 scribed and diakinesis and can not at this time contribute anything further 

 to the question of the origin of the paired chromosomes as they appear 

 in diakinesis. The bivalent chromosomes of diakinesis are sharply de- 

 fined (fig. 24). There is good evidence that here, also, they are arranged 

 in a continuous series. The suggestion is strong that the thick spirem 

 was double and that the diakinetic pairs have arisen by a contraction 

 in length of the material of each chromosome. At the earliest stages of 

 diakinesis it is to be observed that the chromosomes have not completely 

 rounded up, but are drawn out into thread-like extensions which connect 

 the individuals of successive pairs into the double series thus suggesting 

 that, as noted, the pairing is due to the parallel arrangement of two series 

 of chromosomes. 



In diakinesis, as has been almost universally agreed, the pairs of 

 chromosomes lie in the periphery of the nuclear cavity against the nuclear 

 membrane. This position is similar to that of the univalent chromosomes 

 as described above during the early prophases of the somatic divisions 

 (compare figure 24 with 6). This stage in Carex acuta was figured by 

 Juel (1900, fig. 30), but he does not show the doubling of the chromo- 

 somes which is so conspicuous in my preparations. The chromosomes 

 are roughly elliptical with the sides which are in contact somewhat flat- 



