284 Cardiff : Synapsis and reduction 



chromatic character, and are much thicker than when entering the 

 knot. Whether this obscuring of the so-called linin by the 

 chromatin is due to the linin taking on chromatic properties — 

 becoming. chromatin — or whether the chromomeres have increased 

 until they have obscured the linin is impossible to say though the 

 former appears to be the more probable. The thread is not even 

 in outline but is thickened. at short intervals indicating probably 

 the original chromatin centers or chromomeres. From the posi- 

 tion of the nucleolus with reference to the knot it would seem that 

 it must play some part in synapsis, yet it seems to have suffered 

 no change either in size or capacity to take stain as a result of 

 synapsis. However, if the above explanation in regard to the 

 position of the knot is correct, the close proximity of the chromatin 

 and nucleolus may be merely incidental. 



As soon as the threads commence to disentangle from the knot, 

 evidence of a longitudinal fission can often be seen (figures 16-20). 

 Whether this splitting is a separation of the threads which paired 

 in the presynaptic stages cannot be determined for a certainty, 

 although it seems very probable, as many close synaptic knots 

 will have portions of threads on their periphery which always show 

 a paired character. Work of Montgomery ('oo, 'oia f '05), Sut- 

 ton ('02); McClung ('02), Rosenberg ('04^) would seem to indicate 

 beyond a doubt that the reducing division occurs along the line of 

 original fusion. Wilson ('05^, '05&, '06) in his work on Hemiptera 

 showed with special clearness that certain univalent chromosomes 

 — the " idiochromosomes " and the " m-chromosomes " — unite in 



synapsis and the reduction division separates these univalents with 

 their individuality unimpaired. Since this pairing and sub- 

 sequent separation of the moieties has been so clearly proved 

 for certain chromosomes it may not be unreasonable to expect 

 it to occur between all chromosomes. In other words, in Acer 

 there is not a complete fusion of threads in synapsis, but the in- 

 dividual threads probably retain their identity through the syn- 

 aptic stages. 



From synapsis on to the final formation of the heterotypic chro- 

 mosomes it is easy to follow this longitudinal fission. The threads 

 continue to increase in thickness (figures 19 and 20), but the 

 moieties remain in close contact with each other for some time. 



