58 COCCOPHYCES. 
sac, and possessing a diameter of about 1-170", already contains a 
certain number of enlarged gonidia, destined in due time to become its 
own progeny. Not only so, but long before its discharge, and while yet 
it exists as a daughter-cell within tte protecting cavity of the parent 
generation, these selected gonidia are already visible as spots Jarger and 
darker than their fellows. (Plate 22, Fig. 1.) 
“The history of these selected gonidia, as it may be traced ina 
daughter-sphere recently cast forth to seek its fortunes in the world of 
waters around it, is as follows :—The enlarged gonidium is at first a flat, 
thin circular disc, appressed to the internal surface of the sphere, and 
being surrounded by eight of the ordinary zoospores, is derived from 
the coalescence of the two central ones out of a group of ten. (Plate 
23, Figs. 1, la.) Shortly, this disc assumes a more distinctly oval form, 
with a slight constriction across its lesser diameter, in which stage it 
often much resembles a young Cosmarium. (Plate 23, Figs. 2, 2a.) It 
is soon seen to be clearly subdivided into four, and its thickness having 
grown pari passu with its superficies, the group now protrudes into the 
internal cavity of the parent-plant. (Plate 23, Figs. 3, 3a.) Repeated 
subdivision now goes on rapidly (Plate 23, Figs. 4, 4a), till the whole 
body assumes a spherical form, a distinct cell wall being at the same 
time formed, which is revealed by careful illumination, and still more 
clearly by the use of reagents, as a hyaline sphere concentric to and of 
larger diameter than the green one within it, so that there appears to be 
a clear space or ring between the two when seen in section. (Plate 22, 
Fig. 5, 5a.) Finally, the young Volvox consists of a vast number of 
deep green granules closely packed together, and by mutual pressure 
driven to assume a more or less distinctly hexagonal form, and corres- 
ponding in number to the gonidia which are to stud its surface when its 
growth is completed. Shortly hereafter, the whole organism continuing 
to increase in size, clear spaces appear between the gonidia, showing 
that the enlargement of the cell wall and its interspaces is outstripping 
that of the gonidia, which are now approaching maturity. The inter- 
lacing connecting threads are developed simultaneously. (Plate 22, Fig. 
la.) During the whole process the centre of the young Volvox spheres 
continually recedes frum the periphery of the parent, so that when the 
group of young ones has attained the full development of which it is 
capable in this stage they are often pretty cl-sely packed in the internal 
space, and sometimes even slightly deformed by mutual pressure, each 
by this time closely: resembling the parent in miniature, and already 
containing enlarged gonidia of the third generation. (Plate 22, Fig. 1.) 
By this time the clear space originally visible between the gonidia and 
the cell wall has been obliterated, and the cilia may be seen protruding 
through the latter. Some writers state that the daughter-cells rotate 
at this period within the parent cavity. I have frequently seen them 
oscillate so far in one direction and then back to their original position, 
but have never observed a true rotatory motion. Finally, the young 
Volvoces are liberated by the rupture of the parent sac, at a special 
point, clearly marked out for this purpose in its structure. I have not 
met with any observations on this point, but have fully convinced 
myself that it may always be predicted at what point this rupture will 
be effected. 
“The combined action of the pairs of cilia in which the gonidia 
terminate is the actuating power whence proceed both the rotatory and 
the progressive movement of Volvox, and these are both in a definite 
direction. If an imaginary axis be drawn through the sphere, the pro- 
gressive motion being, so to speak, from the north to the south pole of 
that axis, the rotatory motivn is usually from west to east, though not 
always, being occasionally reversed for a few seconds; but for the 
