77 
jet Balfour has shown (A Monograph on the Development of' 
the Elasmobranch Fishes, by E. M. Balfour, M. A. London : 
1878), that the types of the early development of all verte- 
brate animals can be easily derived from that of the typical 
gastrula. 
The general occurrence of a gastrula stage in so many 
widely separated animals is certainly the most pronounced 
feature in embryology, and it is possible that a more complete 
acquaintance with the development and phytogeny of the 
Mollusca may show that the facts held do not, in reality, op- 
pose the view that it is an ancestral form, and the conclusion 
which the facts seem to justify is not that the gastrula theory 
is proved or disapproved, but that our acquaintance with the 
facts must be very much greater than it is at present before 
we shall be prepared to establish any general hypothesis as to 
the ancestry of the Metazoa. 
THE SHELL. 
The two valves of the shell of the oyster originate sepa- 
rately, as I have already stated, while in some other Lamelli- 
branchs the separation of the valves is brought about at a 
later stage of development, by the division of a continuous, 
embryonic shell. 
In Cyclas the embryonic shell makes its appearance as a 
simple, nearly circular cup, which occupies the dorsal mid- 
line of the body, and soon becomes saddle-shaped, and pro- 
longed to form two flaps, which run down into the sides of 
the body. This embryonic shell does not contain any calca- 
reous matter, but appears to be wholly made up of a chitin- 
ous excretion from the cells of the shell area. After it has 
extended out onto the sidfes of the body, calcareous matter be- 
gins to be deposited on its inner surface, at two points, one on 
each side of the body. These centres of calcification grow on 
all sides, and become the calcareous valves of the shell, and 
the flaps of the primitive shell become the epidermic cover- 
ings of the outer surfaces of the two valves. The two centres, 
of calcification grow towards, but not quite up to the medium 
