92 
Hogg, 07i the Water- Snail. 
of an ovoid form, and consisted of a pellucid membrane fdled 
with a transparent fluid, having a very minute yellow spot, the 
yolk, adhering to one side of the cell-wall. Seen with the 
sunlight falling upon it, it had all the brilliant colours of the 
soap-bubble. Viewing it again on the second day, I observed 
that the yolk had a central spot, or nucleolus, rather deeper 
coloured than the rest. On the fourth day the yolk had 
changed its position, and doubled in size, as shown slightly 
magnified (Plate VII. fig. 1). Upon a closer examination, a 
central depression, or transverse fissure, could be seen, which, 
on the sixth day, plainly indicated the line of demarcation in 
the little mass, as represented at fig. 2. From this time it 
commenced to move round the whole interior of the cell, with 
a very slow rotatory motion ; the motion was increased when 
the sunlight shone upon it, from which I concluded, that, as 
it received more heat, its movements were thereby accelerated. 
The increase in size of the two parts of the animal appeared to 
be uniform up to the sixteenth day, when the shell apparently 
occupied the larger portion, represented at fig. 3 ; and the 
spiral axis, around which the calcareous lamellae were being 
deposited, had a much darker colour than the soft, or cephalic 
extremity. On the eighteenth day the tentacle was visible, 
with a small black speck at its root, the eye ; this was seen 
to be protruded with the movement of the tentacle. Upon 
closely watching it, a fringe of cilia could be seen surrounding 
the tentacle and oral aperture ; and, from observing the direc- 
tion of the currents, I am led to believe that the earliest 
rotatory motion is in a great degree, if not wholly, dependent 
upon the action of the cilia. A constant current being kept 
up in the cell-contents, we may conclude, that with this 
motion, we have the conversion of the cell-contents into the 
several tissues ; and probably the whorl-shape of the shell is 
likewise due to the same formative process. The rotation 
was, on every occasion of my observing it, from the right to 
the left, and this always combined with a motion around the 
egg ; the embryo performing a circuit, as represented magnified 
at fig. 4, and forcibly reminding me of M. Wichura's scientific 
investigations into the curious property possessed by the 
leaves of plants, of winding generally in a particular direction. 
He observes : — 
" It is a very remarkable phenomenon, that the circnlarly or heliacally 
acting forces of nature follow an unchanging, definite, lateral direction in 
their course. In cosmical nature the planets describe heliacal lines, wind- 
ing to the right in space, by virtue of their circulation from west to east ; 
since this is combined with the advance, in company with the sun, towards 
a point in the northern hemisphere. In the department of physics we 
meet with allied phenomena in the circular polarization of light, and in 
