Experiments on the Development of the Liver-fluke. 
duced, forming' the eye-spot. Wave-like peristaltic contractions 
pass along the hody from the anterior to the opposite end. In 
the last stage, when the embryo is nearly ready to emerge from 
the shell, it lies slightly curved upon itself at one side of 
the egg, the remainder of the space on the opposite side being 
occupied by the fluid remains of the coalesced yolk-spheres. 
At the anterior end, just beneath the operculum, is a quantity of 
viscid mucus, which forms a sort of lining or cushion to this 
end of the egg. Around the body of the embryo may be dis- 
tinguished a bright border, which is formed by its covering of 
cilia ; these cilia, however, can only in exceptional cases be 
seen in motion before the animal quits the egg. 
The embryo is now ready to come forth ; its movements 
become more marked, and at length a more vigorous extension 
of the body causes the operculum to fly open as if moved by a 
spring. The cap of mucus pours out, the embryo thrusts the 
fore part of its body out of the shell, the cilia begin to move 
instantly the water touches them, and the animal after a 
short struggle succeeds in drawing the whole of its body 
through the narrow opening of the shell, and glides away with 
ease and rapidity through the water. Although light has no 
influence in accelerating development, the embryo itself is- 
sensitive to it. I have repeatedly observed that on removing^ 
a vessel of eggs from the darkened incubator in which they 
were being hatched, only two or three embryos could be seen,, 
but that after it had stood in a window for twenty minutes the 
water was quite nebulous from their presence. 
The form of the embryo is an elongated cone with rounded 
apex, its average length 0*125 mm., its breadth at the anterior 
end 0 027 mm. The broader end or base of the cone is directed 
forwards, and in the centre of this is a short retractile head- 
papilla. The embryo is exceedingly active, and, with head- 
papilla drawn in, swims restlessly through the water. Sometimes 
it goes directly forwards and then rotates on its longitudinal 
axis ; at others, by curving its body, it sweeps round in circles, 
or, curving itself still more strongly, spins round and round 
without moving from the spot. If it comes in contact with any 
object, it stops for a moment as if trying to discover its nature. 
The whole of the surface, with the exception of the head- 
papilla, is covered by long cilia (0'012 mm. in length), which 
are borne by a cuticle composed of five transverse rings of 
flattened cuticular cells about 0'03 mm. long, each with a small 
nucleus 0"00o mm. in diameter. The cilia are of the same 
length over the whole of the cuticle, but on the cells of the 
anterior ring they are more numerous and thus more conspicuous. 
This first row is composed of four cells arranged round the 
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