THE PHASE OF THE NUCLEUS KNOWN AS SYNAPSIS. 595 



whatever for intercellular spaces. The cytoplasm was characteristically sporagenous in 

 being densely charged with fine food granules and containing no vacuoles of measurable 

 size. The nucleus was practically spherical in form and nearly centrally situated 

 (fig. 1). There were two or sometimes three large nucleoli present. The chromatin 

 with its linin appeared as a fine mesh or network of threads. This network, however, 

 is more apparent than real, for, by carefully focusing, one could follow the individual 

 threads passing over and interlacing with one another for considerable distances. The 

 individual threads were finely granular, and, on account of their apparently anastomosing 

 with one another, it was impossible to count or even approximately estimate whether 

 they corresponded to the diploid number of chromosomes. Apart from observing that 

 there were a number of threads of chromatin present, I was unable to distinguish 

 " prochromosomes," which have been recorded as occurring in certain Dicotyledons by 

 Overton (1905) and others. 



The almost spherical form of the nucleus as well as its large size — as compared with 

 that of the vegetative cells — gives the impression that the nuclear sap is exerting a con- 

 siderable osmotic pressure upon the nuclear membrane. In this connection it should be 

 remembered that in the cytoplasm there are no vacuoles in the sense that we have them 

 in the growing vegetative cells, the entire cytoplasm being charged with food substances, 

 and yet these sj)ore-mother-cells are actually growing in size. This circumstance, 

 together with the turgid appearance of the nucleus, makes it difficult to escape from the 

 conclusion that the nuclear cavity is acting as a vacuole. That the enlargement of the 

 nuclear cavity is brought about by the increase in the amount of its contained fluid 

 there can be no doubt, and an increase of osmotic pressure acting upon the nuclear 

 membrane would necessarily follow. This finds an expression in the stretching of the 

 nuclear membrane and a considerable increase of the nuclear space. The nuclear cavity 

 thus acts as a vacuole in exerting a great internal osmotic pressure, and, in doing so, 

 facilitates growth in a cell where ordinary vacuoles are absent from the cytoplasm. 

 This seems a plausible explanation for the great size of the nuclear cavity of spore- 

 mother-cells. And, as we shall point out, the growth of these cells proceeds with the 

 increase in the size of the nucleus. 



In fig. 2 we have represented a stage where the increased amount of nuclear sap 

 shows itself by a slight distension of the nuclear membrane beyond the area occupied 

 by the chromatin. On the lower side of the figure one sees a clear space — appearing as 

 a narrow crescent in section — between the chromatin and the membrane. This, at first 

 sight, might be mistaken for a shrinkage of the chromatin, but the comparatively 

 even and regular surface of the chromatin area, as well as a consideration of the 

 relative size of the latter in the preceding fig., makes it quite evident that the clear 

 crescent-shaped area is due not to shrinkage or contraction, but to an increase in the 

 nuclear sap with the consequent distension and withdrawal of the nuclear membrane 

 from the chromatin. 



In fig. 3 is represented a slightly later stage, where the distension of the membrane 



