114 NEW SOUTH WALES 
By mixing the male and female fluid it has been found perfectly easy 
to fertilize the ova. After a period varying from one to two days the 
eggs are hatched; a minute free-swimming organism is the result. By 
the aid of warmth and many favourable circumstances eggs may be 
hatched in twohours. A somewhat warm temperature in the sea is the 
best for hatching, and the slightest cold of a severe character destroys 
the ova. Our climate should be about the best for oyster culture, and 
as far as the rock oyster is concerned the more abundant it becomes as 
it is traced northwards and into warmer seas. About twelve hours after 
the free-swimming stage the shells begin to appear, with the germs of 
other organs. The shells are distinct from the first, and for a long time 
have a smooth rounded outline. At the end of six days the shells 
nearly cover the embryo, which is hardly more than visible to the 
unassisted eye as the merest grain. All the larger organs can be seen 
with a microscope, as they are then separated, with the exception of 
course of the reproductive tissues such as ovary and milt. After 
swimming about in this state for a few days, the period of which has 
not. been exactly ascertained, the young oyster, the so-called ‘ spat,” 
fastens itself by one valve of its shell to any rough and clean body. It 
is then very thin and delicate, but grows fast. If it has not some object 
to fasten to it sinks to the bottom. There it may find the living and 
dead shells of its relatives or a rock or other shells, and there it will grow. 
It is not uncommon to find nearly a hundred small “spat” on one 
oyster shell. Itis then that oyster cultivators collect them and lay 
them in convenient places to fatten. Without artificial assistance there 
can be no doubt that the great mass of oyster ova perish, 
Many observers have asserted that the ova are fertilized within the 
ovary and ‘are afterwards nursed in the folds of the mantle. Dr. 
Brooks has proved that this is not true of the American oyster (0. 
virguviana), and it is probably untrue of all. I have carefully examined 
many ripe female oysters, and never could find any embryos on the 
mantle. It is probable that the egg is discharged into the water, to be 
fertilized, where in oyster-beds and amidst its own species it easily may 
meet with a male cell. If however it does not meet with such, the ova 
are rapidly destroyed by sea-water. The fertilized ova crowd toge- 
ther on the surface as a thin scum, where they no doubt fall a prey 
to many enemies, small ones of course, because they are too minute 
themselves to form an article of food to any but very little creatures. 
Still the chances of their escape from accident are not in their favour. 
This is why preserves and ponds must be so advantageous to their culti- 
vation. J£ the collection of the spat is found to be so profitable, how 
much more would properly secluded places be where the ova could be 
left in tranquillity with all the chances of life in their favour. Artificial 
fertilization is not wanted,—all that is required is the protection of hatched 
spawn. Moébius* estimated the average number of eggs laid by O. edulis 
at over a million (1,012,925). Eyton rates this higher, and says that 
there are about 1,800,000.t Dr Brooks calculates from a much more 
accurate series of observations, and estimates the ova from the largest 
Virginian oyster as about 60 million, or for an average one about ten 
eee Die Austern und die Austernwirtschaft,” von Karl Mobius, Berlin, 1877. 
+ ‘History of the Oyster and Oyster Fisheries,” by T. C, Eyton, London, 1858 
The figures above are given at p. 24, 
