Williamson on Volvox glohator. 
49 
ample no cells were at first visible ; but as I watched them, they 
came gradually into view in some parts of the organism, but 
not in others. Fig. 11 represents a portion of it, in the upper 
part of which the cells are visible, whilst in its lower part 
they cannot be traced ; here the tissues were transparent and 
apparently quite structureless, as was the entire sphere in the 
first instance. The transition from the one condition to the 
other was gradual ; the dark lines becoming less conspicuous 
and finally disappearing as we approached the opposite side of 
the Volvox to that on which they were most distinct. This 
specimen illustrates thousands that have been examined, and 
proves that the apparent absence of the cells from so many of 
the objects is no proof that they do not exist, but merely shows 
that certain favourable conditions are required to bring them 
into view. We are justified in concluding that they exist alike 
in all the specimens of Volvox, and are not merely accidental 
developments in a few individuals. 
In my mounted preparations we obtain further evidence 
respecting the nature of these two membranes — the proto- 
plasmic and the cellular. We see that the wall of the sphere 
has an appreciable thickness, the inner margin being as 
definite as the outer one, and nearly parallel with it. Fig. 5 
represents this as seen in a section of a Volvox. A moment's 
inspection of my preparations would convince the most 
sceptical that such is the case ; several of these peripheral 
cells, as seen in the section, are more highly magnified in 
figs. 14 and 15. The thin investing pellicle (15 d) com- 
presses the outer wall of each cell into conformity with the 
peripheral curve of the sphere; laterally the septa (15 a) are 
straight and parallel to one another. Internally, each cell is a 
little turgid (15 «), the centripetal pressure at this point being 
obviously at the minimum, and allowing the primary tendency 
of the cell to assume a spherical form to manifest itself. The 
green protoplasm adheres firmly to the peripheral wall of each 
cell, through which the cilia are protruded. Between the 
true cell-wall and the protoplasm, we have the protoplasmic 
membrane (14 and 15 cc') in variable conditions. Some- 
times it forms an oval cell (14 c'\ sometimes it is oval at one 
end and flattened out at the other (14c, 156*') ; at others it is 
not only flattened out at each extremity and in close opposi- 
tion with the cell- wall, but even at the two sides (as in the 
centre cell of fig. 15) the two membranes are closely ap- 
proximated. 
If we turn to a superficial view of the same specimens, we 
shall obtain similar results. It must be borne in mind that 
in the living Volvox, even where the cells are visible, we only 
e 2 
