by the Method of Dark-ground Illumination. 619 
unaltered in structural appearance. The protoplasmic threads or fibrils, 
which in the figure drawn are of fairly large size, seem to be formed by the 
pulling out of the outer layer of the protoplast, the fine colloid structure of 
this layer being very well seen at the bases of any of these threads which 
could be observed in optical section in this region. The nature and structure 
of the fibrils are further described below (sect, b). The particles in the outer 
layer were in rapid movement, and nothing could be seen of a hydrogel 
layer (S chick t) bounding this on the outside. Some definite boundary, 
however, whether a mere surface maintained by surface tension (molecular 
attraction) or a definite membrane, must be assumed as the colloid particles 
do not escape into the surrounding liquid. 
PI. XLII, Fig. 5, shows much the same general features. The cell was 
here plasmolysed with an electrolyte, a 10 per cent, solution of potassium 
nitrate. The layering of the protoplast is much the same as before. Large 
processes are drawn out in the case shown, which pass into coarse fibrils at 
the end and there generally branch repeatedly. The fine structured surface 
layer appears much as before, but the fibrils are much thicker than in PI. XLI, 
Fig. 1 . On the whole no marked difference can be said to occur in the 
appearance of the two cases, typical examples of many, one plasmolysed 
with a non-electrolyte and the other with an electrolyte. 
PI. XLI, Fig. 3, shows a type of plasmolysis frequently obtained with 
practically any of the agents used — glycerine, potassium nitrate, cane 
sugar, sodium chloride, and so on. The formation of vesicles of the 
nature shown is well known in Spirogyra, and has been fully described by 
de Vries (’ 85 ) and others. The vesicles are thin-walled ‘ bladders , of proto- 
plasm containing presumably cell sap. The figure is drawn to show, as 
nearly as possible, the different appearances observed in three depths of 
focus, (a) is focused so that the surface and edge of the vesicle wall are 
seen ; {b) shows the contents of the vesicle and the general mass of the proto- 
plast ; while (c) is in the upper focus, just within the wall of the cell. 
The wall of the vesicle has the appearance of a slightly milky or 
opalescent elastic membrane, with some extremely fine particles in its sub- 
stance which oscillate rapidly with a large free path when seen in surface 
view. At the edge, as seen in optical section, these particles naturally 
appear more crowded. If the wall of the vesicle contains any protoplasm 
in the hydrogel state it must be imagined as a very thin layer (too thin to 
show the usual hydrogel structure) both on the outside and inside of the 
vesicle. The layer between remains in the hydrosol state. It is certain 
that when the vesicle undergoes fixation the whole is converted into a gel, 
with the usual opaque and heterogeneous appearance. The vacuole then 
seems to emerge from the inner portion of the protoplast, and there is 
thus evidence to show that the vesicle is produced from the layer with fine 
particle structure, which almost certainly normally exists on the inside of 
