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POPULAR SCIENCE REVIEW. 
suppose, ejected in squirt-fashion, from the parent : their 
development and growth proceed gradually, and as they — 
series by series — leave the ovary, they pass to the outer 
gill-folds, which play the part of a uterus. Here they are 
retained in the various compartments, and, by the action of 
the cilia, the zoosperms are introduced, and thus the ova 
become fertilized, and are afterwards slowly extruded.* 
Let us trace the further development of the embryo. Just as 
the yolk is about to undergo segmentation, we observe two 
portions of it set apart : these do not suffer division, but are 
soon changed into as many three-sided valves, whilst the 
remainder of the mass is converted into a small roundish body, 
recovered with vibratile cilia, by means of which it is caused 
to move slowly round and round. As soon as it leaves the 
parent and can be examined, it is then seen that the cilia are 
not indiscriminately disposed, but are arranged upon a sort of 
pad or fleshy cushion, which projects from between the valves. 
This peculiar apparatus, which was discovered about ten years 
ago by M. Davaine,f is provided with several muscles, by 
which it can be extended or retracted, at the will of the animal. 
Its construction and position are well shown in the accompany- 
ing delineations (PI. XVIII., figs. 3, 4, 5, and 6). What is the 
use of this strange mechanism ? It is to admit of the distri- 
bution of the species which, were it absent, would be for ever 
confined to one locality, and the result of this would be star- 
vation of the colony. The young oyster swims away when it 
leaves the parent ; but it soon tires of its adventures, and 
after a couple of days it settles down into respectable oyster- 
hood, and commences life on its own account. Its “ swim- 
ming belt ” now becomes atrophied, and disappears, and our 
protege attaches itself to some solid body, where it remains 
for the rest of its days. When it is fifteen days old, it is 
about the size of a small pin's head, and has the form 
represented in the engraving (PI. XVIII., fig. 11), which 
also shows its gradual increase of size, till it is twelve 
months of age. Ere concluding our remarks on this branch 
of the subject, we may mention that, in the opinion of Mr. 
Eaton, who has carefully studied the development of the 
oyster, the production of ova — which in some cases extends 
* Vide Siebold, loc. cit., and Kemmerer, loc. cit., p. 17. 
f “ Recherches sur la Reproduction des Huitres,” Comptes Bendus, 1855. 
M. Coste, in his fine memoir, gives a brief description of the process de- 
scribed by M. Davaine ; but we fancy Mr. Ashworth has mistaken the 
author’s meaning in the following passage, where, speaking of the ciliated 
pad, he writes, — “ d Vaide desquels V animal pent , d volonte, le faire sortir hors 
des valves ou Vy faire rentrer” 
