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shells of their own ; they leave the parent at a tolerably advanced stage of 
development, and the period of their free existence is very short. Mr. W. 
K. Brooks states that in the American oysters things go on very differently. 
Finding that no young oysters were to be met with in the mantle-cavity of 
the parents, he tried artificial fertilization of ova taken from the ovaries, and 
was completely successful, — raising millions of young oysters, and tracing 
them through all their stages of development up to the time when they had 
acquired all the characteristics which Salensky, Lacaze-Duthiers, Mobius, 
and others, have figured and described in the young European oyster at the 
time it leaves its parent. On the other hand, he never found young oysters 
inside the mantle-cavity of an adult, although from the state of the ovaries 
the individuals examined were evidently engaged in spawning. Mr. Brooks 
gives the following statement of the general results of his investigation : — 
1. The oyster is practically unisexual; at the breeding season each in- 
dividual contains either eggs or spermatozoa exclusively. 
2. Segmentation of the egg takes place very rapidly. 
3. Segmentation is completed in about two hours and gives rise to 
a gastrula, with ectoderm, endoderm, digestive cavity, and blastopore, and a 
circlet of cilia or velum. At this stage of development, the embryos 
crowd to the surface of the water and form a dense layer, less than a quarter 
of an inch thick. 
4. The blastopore closes ; the endoderm separates entirely from the ecto- 
derm, and the two valves of the shell are formed, separate from each other, 
at the edges of the furrow formed by the closure of the blastopore. 
5. The digestive cavity enlarges and becomes ciliated, and the mouth 
pushes in as an invagination of the ectoderm at a point directly opposite that 
which the blastopore had occupied. The anus makes its appearance close to 
the mouth. 
6. The embryos scatter to various depths, and swim by the action of the 
cilia of the velum. The shells grow down over the digestive tract and 
velum, and the embryo assumes a form so similar to various marine 
lamellibranch embryos which are captured by the dip-net at the sur- 
face of the ocean, that it is not possible to identify them as oysters without 
tracing them from the egg. The oldest ones that he succeeded in rearing in 
aquaria were exactly like the embryos of Cardium, as figured by Loven. 
7. The ovaries of oysters less than one and a half inch in length, and 
probably not more than one year old, fertilized with seminal fluid from males 
of the same size, developed normally. 
An illustrated report on these highly important and interesting investiga- 
tions will appear shortly in the Report of the Maryland Fish Commission 
for 1879. — Sillimari’s Journal , December, 1879. 
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