54 
Williamson on Volvox glohator. 
The origin of the superficial pellicle (fig. 14 c? and Ih d) 
remains to be considered. In my last memoir, I stated that 
each young gemma was developed within a large transparent 
vesicle (fig. hf and 6 f), which appeared to be the expanded 
cell-wall of the primary cell (a). My more recent investigations 
confirm this conclusion. When we detach a young gemma and 
its vesicle from the parent Volvox, the vesicle usually carries 
away with it a few of the contiguous protoplasms adhering to 
its outer surface (fig. 6 6), indicating the firm adhesion between 
this vesicle and the walls of the sphere, which we know to 
exist. This vesicle expands as the gemma increases in size. 
At first, the latter is closely invested by the former ; but when 
the cilia are developed on the surface of the young organism, 
the vesicle becomes considerably distended, allowing the 
gemma to revolve freely within its prison house. (See fig. 5.) 
At this time the gemma is already invested by its proper 
superficial pellicle ; hence the latter cannot be the modified 
primary germ-cell, as supposed by Mr. Busk, but is a new 
growth developed on the surface of the gemma whilst enclosed 
within the enlarged germ-cell. The source of this pellicle must 
be sought for in the aggregated protoplasms. It appears to be 
an independent secretion thrown off by them, in the way that 
the epidermal cells of a leaf co-operate to produce the similar 
structureless superficial pellicle. If this be a true homology, 
it would countenance the opinions of Schleiden and Payen, 
rather than of Mohl and Henfrey, the latter of whom regards 
the superficial pellicle as composed of the altered primary 
walls of pre-existing cells, and not as an external secretion. 
Henfrey 's explanation is that which I applied to the pellicle 
of Volvox, until the suggestion of Mr. Busk, respecting the 
primary condition of the volvocine protoplasms, showed me 
that in this instance the hypothesis was untenable. 
The relative periods at which the cells, the superficial 
pellicle, and the cilia make their appearance is not easily 
determined. So far as I have been able to form an 
opinion, I am disposed to think that the cilia first make their 
appearance, the cells and the outer pellicle being subsequent 
growths. A priori^ we should have expected this to be the 
case, since it would not have been easy for delicate, flexible 
cilia to force their way through one or two imperforate invest- 
ing membranes. When the cilia are first produced they are 
very short, but they gradually lengthen, apparently by addi- 
tions to the base of each, secreted by the respective proto- 
plasms. After their formation we can readily understand how 
tlie pellicle could be secreted from the mass of protoplasms 
between and round the roots of these cilia, no pellicle being 
