VOLYOCINE. 61 
only made evident by the action of reagents. (Plate 22, Fig.2.) He 
regards the globe of Volvox as a ‘hollow vesicle, the walls of which 
consist of numerous angular cells filled with green endochrome, &c., the 
intercellular spaces being more or less transparent,’ and the ciliated 
zoospore as representing the endochrome of a cell having two walls, the 
internal one being separated from the outer cell-wall, except at a few 
points where it is retained in contact by the connecting filaments, and 
the external one forming the hexagonal divisions on the surface. He 
further holds that the periphery of the sphere, when seen in section, 
has an appreciable thickness, its inner margin being definite and parallel 
to the outer one; and that the sides of the hexagons being continued 
downwards through the thickness of the outer membrane, the appear- 
ance of all these structures, if they could be seen simultaneously, would 
be that shown in Plate 23, Fig. 6. 
“Even in deeply stained specimens I have never been able to detect 
the existence of these hexagons as other than an entirely superficial 
structure, and at present my impression is that the hexagonal structure 
has a different significance. 
“Tn the very early stage of Volvox-life the embryo gonidia are 
encased in a distinct transparent outer-sphere. (Plate 23, Fig. 5,5a.) At 
a later period. owing to the more rapid growth of the gonidia than of 
the case, the latter closely invests the former, which are, in fact, 
embedded in it. In the next stage, if not in the earlier condition, by 
the continued growth of the gonidia at a greater rate than that of the 
containing sphere, they are so closely appressed as to assume the hex- 
agonal form, and the interstices must of necessity consist of a thin film 
of the substance of the containing spherical envelope, moulded, so to 
speak, into corresponding forms. But now the diameter of the young 
Volvox, which is by this time sent forth on its independent career, 
rapidly increases, the gonidia assuming their spherical or pyriform 
shape as their mutual pressure diminishes, and being hourly separated 
by greater intervals, If, now, the actual formative matter of the sphere 
receives no further or only a disproportionate increment, but is 
gradually attenuated by continued expansion, as a soap bubble is dis- 
tended by blowing into it, the hexagonal lines into which it has been 
moulded by the previous mutual pressure of the embryo gonidia will be 
gradually stretched in all directions into finer proportions; and just as 
this figure is that which is necessarily assumed by a number of spherical 
bodies under mutual pressure, so the most economical disposition of this 
particular part of the Volvox-structure will necessitate ita constant 
attenuation into hexagons of ever-increasing delicacy. (Plate 22, Figs. 
1,3; Plate 23, Fig. 7.) If the process be continued long enough, it 
may finally result in the structure becoming too filmy to be detected by 
any microscopical observation ; and it is worth noticing that it is 
usually in spheres of small or medium diameter that the hexagonal 
divisions can be developed, and not in those of the largest size. Such 
appears to me at present to be the rationale of the formation of this 
structure, 
“The internal cavity of the sphere is said to be filled with a ‘ muci- 
laginous fluid.’ ,If a Volvox be ruptured under a cover-glass, and aniline 
purple introduced by capillary attraction, the colour seems to be for a 
while repelled at that part which is in front of the rapture, and to flow 
round it on either side. It is only after w considerable time that it 
gradually penetrates this space, and brings out, by staining it of a deep 
purple tint, a mass of hazy matter, from which proceed streaks or 
lines radiating more or less regularly from its south pole. This structure, 
to which I do not think attention has been hitherto called, is also some- 
times developed in deeply stained specimens within the slightly 
K 
