THE PHASE OF THE NUCLEUS KNOWN AS SYNAPSIS. 597 



imperfect fixation or to the fact that the sections were not cut in a median plane through 

 the nucleus. It should perhaps be pointed out that the larger the nuclear cavity becomes, 

 the more osmotically sensitive become the substances within the limiting osmotic 

 membrane. Consequently this stage offers perhaps more difficulties for proper fixation 

 than any other stage in the history of the nucleus. This was made manifest in several 

 lots of material which were fixed in Fleming's strong solution. It was found that 

 whenever this strong osmic acid solution was used for fixing, the cytoplasm appeared 

 more or less plasmolysed and the chromatin mass within the nuclear membrane showed 

 a corresponding amount of shrinkage. In this same material, however, the early stages 

 of the mother-cells — before the increased size of the nuclear cavity — showed perfect 

 fixation. It would therefore seem that the very much enlarged nuclear cavity favours a 

 shrinkage of the cytoplasm as well as of the chromatin , and this condition of the latter 

 might easily be mistaken for a natural contraction. In working out the present series 

 every slide was rejected that showed the slightest trace of plasmolysis. 



As stated above, certain sections were passed over because they were not cut 

 in a median plane through the chromatin mass. In this connection it might be well to 

 point out that when the nucleus reaches its full size it may be 25 m or 30 m in its 

 longest diameter, and, cutting sections of an anther at this time, we invariably obtain 

 numbers of sections of cells that do not pass through the median line of the nucleus. 

 Some of these sections may show a very large nuclear cavity and a very small shaving 

 of chromatin at one end. Such small areas of chromatin at first suggest a considerable 

 contraction, and they were frequently found. This fact is mentioned here because I 

 feel sure that some of the figures that have been published to show the synaptic con- 

 traction have been drawn from such oblique sections. Fig. 13 is inserted here to 

 illustrate this point. It was drawn from a section that was not median. All of the other 

 figures were drawn from sections that were cut in a median plane — or nearly so — through 

 the chromatin mass. Now, if we examine all of these stages from figs. 1 to 1 2 in series, 

 we find no diminution whatever in the area occupied by the chromatin. At the same 

 time it is obvious that the nuclear cavity has increased to nearly twice its original size. 

 As we show in figs. 16, 17, 18, 19, and 20, this enlargement of the nuclear cavity 

 gradually continues until it reaches a cubical dimension which is quite three times that 

 shown in fig. 1. But during this entire period of growth there is not the slightest 

 evidence that the chromatin mass has shrunken or contracted. The obvious conclusion 

 I draw from this is that, owing to the greater osmotic pressure caused by the increase of 

 the karyolymph, the nuclear membrane has distended and withdrawn from the chromatin 

 mass. The latter remains in its original position and has not moved away from the 

 membrane, as has so frequently been stated by so many writers. 



Now, while no actual contraction could be detected in the area occupied by the 

 chromatin during this period of growth of the nucleus, the threads of the chromatin 

 undergo a distinct modification. In the very early condition shown in fig. 1 the 

 chromatin appears as a distinct but irregular network. This network or mesh is corn. 



