60 COCCOPHYCE. 
young plants, and on the other, either to destroy the vitality of the 
whole colony or to drive it to seek refuge in deeper water. 
“ A curious instance of this sensitiveness to varying conditions of light 
and heat occurred to myself. I had two shallow vessels in a north 
window, each containing a goodly supply of Volvox. Cold and inclement 
weather, which prevailed for weeks together, seemed to check their 
increase, for I found but few young spheres from day to day among the 
older ones. Thinking that a moderate degree of warmth would tend 
to increase my colony, I transferred one vessel, fortunately not both, to 
the floor of a warm greenhouse. In forty-eight hours all were dead, and 
in a few days scarcely a vestige remained of the countless corpses which 
had copiously strewed the bottom of the glass. 
“We must now revert to the minute structure of the mature parent- 
sphere, which has been exhaustively stadied by Cohn, Busk, and 
Williamson. 
“In the outset it should be stated that the last-named observer 
believes that there are two distinct forms of Volvox, in one of which 
the peculiar structure which I am about to describe exists, while it is 
absent from the other. Busk disputed the accuracy of Williamson’s 
observations on this point, but in an appendix published subsequent to 
the body of his essay he states that he has detected this same structure 
in specimens from Manchester, but not in his own. 
“T have failed to develop it by the means recommended by William- 
son, but have succeeded in making it evident enough in a great number 
of specimens from Sntton, by the use of these reagents, and especially 
by the application of aniline purple, an invaluable auxiliary in the 
examination of minute vegetable cell-structures. 
“ This substance stains the protoplasmic elements of such structures 
to a colour which appears deep purple by direct light and crimson by 
dark background illumination, and reveals details which are wholly 
invisible without its use. 
“The colour is, however, greedily absorbed by some of the materials 
used by the microscopist, so that a judicious choice of these is necessary 
to ensure success. Objects stained in this manner are, for instance, 
rapidly bleached if mounted in gold-size cells, and I have for the present 
adopted zinc-white in its place. Among other reagents which I have 
used are eosin, iodine, iodised glycerine, carmine solution, potassium 
permanganate, nitrate of silver, and other salts, some of which bring 
into view various parts of the minute structure of plants; but aniline 
colours, applied with due precautions, produce the most rapid and 
striking effect. 
“Professor Williamson describes the structure in question as a net- 
work of lines dividing the whole surface into hexagons, in the centre of 
each of which is seated one of the gonidia. 
“The delicate ‘protoplasm-threads’ proceeding from each of these 
to its six surrounding neighbours never pass-through the angles of the 
hexagons, but always through the side of each hexagon to the next 
gonidium. (Plate 23, Fig. 3.) Hence it appears that ‘the points of 
adhesion are chosen prior to the development of the outer cell 
membrane,’ in which light Williamson regards the hexagonal division. 
In his specimens this structure was developed by immersion in glycerine 
for some time. I have failed to obtain more than the faintest sugges- 
tion of it by these means, but it is often brought out by the application 
of aniline purple, as is also an important detail shown in drawings 
made from his preparations, viz., that at the angles of the contiguous 
hexagons there is sometimes a distinct doubling or separation of the 
lines, whence he concludes that each side of the figure is really formed 
by two delicate cell-walls in close juxtaposition, the duality of which is 
