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Transactions of the Royal Society of South Africa. 
embryonic cell-mass. By the fourteenth day the embryo had become 
quite distinct : the segmentation cells had lost their outline, and the remains 
of yolk-cells were scattered round the embryo, which now almost filled half 
of the egg-case and extended from pole to pole. 
On the twentieth day the embryo, the future miracidium, began to show 
slight movements of contraction and subsequent expansion. Evagination 
and invagination of the future alimentary canal were distinctly seen, accom- 
panied by the ingestion of food-products into the embryonic digestive 
tract. The digestive tract could be clearly seen, on account of its lumen 
being filled with coarse granules. 
On the twenty-third day the embryo had so much increased in size 
as to lie with its mouth right up against the operculum. The wriggling 
movements, the nickering vibration of the cilia, indicated that it had now 
merely to lift the operculum to emerge from the egg-case as a free-swimming 
miracidium. As soon as the eggs were now exposed to sunlight, the mira- 
cidium burst open the operculum and soon escaped to swim about in 
a smooth, swift-gliding fashion. It is, of course, well known, from experi- 
ments carried out by Leuckart, Thomas, Looss, and others, that sunlight is 
necessary for the miracidium to finally break through the operculum, and 
thence swim about freely in search of the intermediate host. 
I now introduced a snail into the dish containing the miracidia. At 
first they swam about aimlessly, but as soon as they happened to come to 
about half an inch or so from the snail, they suddenly changed their course, 
swam straight up to the snail, and began to attach themselves to the exposed 
parts. Some soon let go their hold, swam away, again approached the snail, 
and attached themselves to it. Others, meantime, crawled over it with the 
definite object it appeared of entering somewhere. It was apparent that 
sooner or later some would find their way through the pulmonary aperture 
into the mantle cavity, encyst themselves round or in the kidney, or work 
their way up the visceral mass and encyst themselves in the liver. 
As Looss (15) had as early as 1896 completely worked out the life- 
history of Paramphistomum cervi in Egypt, where Physa alexandrina 
(Bourg.) and Physa micropleura (Bourg.) are the intermediate hosts, I 
did not consider it worth while to actually infect snails for the purpose 
of my investigation. He found (loc. cit., p. 186) that after a period of 
fifteen days the sac-like sporocysts contain immature rediae, and about 
fifteen days afterwards the first generation of rediae appear. 
The Rediae and Cerceriae. 
The rediae are colourless transparent organisms containing daughter- 
rediae in various stages of development. The more developed daughter- 
rediae are confined to the anterior region of the parent redia. The birth- 
