THE PHASE OF THE NUCLEUS KNOWN AS SYNAPSIS. 599 



thickening, and this is brought out in figs. 18, 19, 20, and 21 ; but they never lose the 

 evidence of their double nature. In most cases at this time the double threads showed 

 the presence of disks or short segments — the chromomeres (Allan, 1905). These are 

 indicated quite clearly in figs. 19, 22, 23, and 24. 



In the stages represented by figs. 25, 26, and 27, the threads of the spireme have 

 become so much differentiated that they are easily identified as definite chromosomes. 

 They have become so much separated from one another that they appear evenly dis- 

 tributed throughout the very much enlarged nuclear cavity. Bach chromosome may 

 be followed from end to end without difficulty. 



There are, then, during this growth period of the nucleus, certain interesting and 

 important changes in the nature of the chromatin threads, but I find no evidence 

 whatever that the chromatin has contracted. My interpretation of synapsis is 

 simply that it represents a "growth period" of the nucleus — a period daring which 

 there is a great increase in the amount of nuclear sap which results in a distention 

 and withdrawal of the nuclear membrane from the chromatin. The question that 

 naturally arises is, Why should this growth period occur in the life-history only at a time 

 immediately preceding the reduction division ? My answer to this is suggested in 

 certain statements made in the early part of this paper in regard to the contents of 

 these mother-cells. Each one of these cells is charged with sufficient food substance 

 for the production and sustenance of four spores. In being so charged they are really 

 storage cells — storage cells that have the power of merismatic activity. Moreover, 

 this merismatic activity is not of the ordinary kind, for it finds an expression in two 

 divisions that follow one another very rapidly. Then, again, these cells exhibit a 

 marked power for rapid growth. Now, nowhere else in the life-history do we find cells 

 with such active and varied properties. It is not surprising, therefore, that we find 

 they differ in their constitution from ordinary vegetative cells. One striking difference 

 is the absence of vacuoles from the cytoplasm. In ordinary growing vegetative cells 

 there is a great production of cell-sap. This accumulates in the vacuoles and generates 

 an osmotic pressure which facilitates growth. In these growing spore-mother- cells, 

 on the other hand — which are both storage and merismatic in character — there are no 

 vacuoles of any measurable size, and consequently there are no open bodies of cell-sap 

 accumulated in the cytoplasm. There is, however, a great body of sap accumulated 

 within the nucleus, which gives the latter its characteristic appearance and distinguishes 

 it at once from ordinary vegetative nuclei. It is, of course, out of the question to 

 prove by actual experiment that the enlarged nuclear cavity produces an internal 

 osmotic pressure, and distends the cell in the same manner that the vacuole is believed 

 to do in the case of vegetative cells. The distended and turgid condition of the 

 nucleus during this period of growth, in addition to its great size, is, however, sufficiently 

 convincing that the pressure set up by the nuclear cavity is just as great as that set 

 up by a vacuole of like dimensions. It is quite common, even among vegetative cells 

 that have been especially differentiated for storage or secretion, to find the nuclear 



TRANS. ROY. SOC. EDIN., VOL. XLVII. PART III. (NO. 20). 88 



