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XX. — The Phase of the Nucleus known as Synapsis. By A. Anstruther Lawson, 

 Ph.D., D.Sc, F.L.S., F.R.S.E., Lecturer in Botany, University of Glasgow. 

 (With Two Plates.) 



(MS. received November 14, 1910. Read December 19, 1910. Issued separately January 26, 1911.) 



That phase of the nucleus just preceding the heterotype mitosis, and which was 

 first called Synapsis by Moore (1895), has come to be regarded as an important and 

 critical period in the life-cycle. It was called synapsis because of the apparent con- 

 densation and contraction of the chromatin at one side of the nuclear cavity. This 

 discovery at first received scant support, the synaptic contraction being considered by 

 many investigators to be nothing more than an artifact — an artificial contraction caused 

 by imperfect fixation. In 1897, however, Moore's discovery was confirmed by Miss 

 Sargant, who not only observed it in fixed material, but also found it in the living 

 pollen -mother-cells of Lilium. 



Since that time the importance of this apparently contracted condition of the 

 chromatin has steadily grown with the increase in our knowledge on the subject of 

 reduction division, so that many of the eminent and leading cytologists of the day 

 have come to interpret this phase known as synapsis as that critical period when the 

 actual fusion and consequent reduction of the chromosomes take place. It is believed 

 by some to represent the actual blending of the maternal and paternal chromatin. As 

 one writer expresses it, " It is the critical stage in the history of an organism. It is the 

 end-result of fertilisation." 



That such a stage exists, I think, there can be no question. It has been observed 

 and described by too many careful and eminent workers to warrant a denial of its 

 existence. These observations not only come from every botanical centre where cyto- 

 logical investigation is carried on, but they have been made upon representative types 

 of all the main groups in the plant kingdom. 



In her work on Lilium, Miss Sargant (1897) tells us that "the approach of synapsis 

 is first indicated by the appearance of drops of nuclear matter adhering to the chromatic 

 network. ... A little later the nucleoli lose their well-defined outline, the nuclear 

 membrane becomes indistinct, and the chromatic threads show a tendency to collect 

 round the nucleoli at one side of the nuclear cavity. . . . The nucleus of the pollen- 

 mother-cell has now entered on the period of contraction called synapsis which precedes 

 the formation of the spireme threads." 



In Allen's (1904) account of this stage in Lilium canadense we find the statement 

 that "the fusion of the threads and of the chromosomes occurs very early in synapsis ; 

 but after the fusion the synaptic condition persists, certainly for days, perhaps for weeks 

 or more. Towards the end of this period, the aggregation of the spireme becomes 

 gradually looser, and then follows a stage in which the thread is very evenly distri- 



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