Zoology. 283 
sequently introduced, itself forms the cellular contents, fills the cavity 
of the new cell, may there be applied to different prolonged functions, 
‘may live longer than the cell itself, or remain in reserve in the cavity 
of the latter, to serve the further purposes of nutrition or generation of 
new cells. The egg, in the two united vesicles of which it is compos- 
ed, presents us with striking examples of a survival of the matter 
which has served as a generating centre, because we there see the ger- 
minative corpuscles persist when the parietal membrane which encloses 
them is dissolved, and take part in the new formations after it has been 
completely absorbed. The yolk there survives the vitelline membrane, 
_and whilst the latter is gradually destroyed from the earliest period of 
its development, we see the yolk continue to nourish the embryo until 
after its birth. 
Such are the grave, numerous and decisive objections, which arise 
against a doctrine, which must rather be considered as a bold inven- 
tion of the mind than the carefully considered expression of satisfac- 
tory observation; but although the bases of this doctrine are uncertain, 
it has not the less rendered an eminent service to science, because in 
he commencement it gave rise @ priori to the conception of the pos- 
sibility that cells might be developed around a centre ; and its influence 
as been very great in directing observers in a fruitful path, and in ex- 
citing important researches, among which we may mention those of 
Valentin, Vogt, Bergmann, Reichert, Bischoff, Barry, Lebert and Henle. 
In my turn I shall make known the result of the observations which I 
have made upon so disputed a subject—observations which during sev- 
eral years have been many times detailed in the course of instruction 
which I give in the College of France. 
2. Microscopic Structure of Shells.—An elaborate article on the 
microscopic structure of shells, by W. Carpenter, opens the volume of 
Reports of the British Association for 1844. It is illustrated by twenty 
plates, containing enlarged views of sections of various species of shells. 
The various facts ascertained are considered as proving that the calce- 
rous matter is secreted within animal cellules, and owes its peculiar struc- 
ture to this mode of formation. Many shells broken across have what 
appears to be a crystalline fibrous structure. This is eminently the case 
with the Pinna. But Mr. Carpenter shows that each minute prism is 
encased in animal membrane within which it was secreted ; and by so- 
lution in acid, he obtained the animal portion having the same basalti- 
form character as the shell. Ina transverse view magnified, the sur- 
face appears finely tesselated with polygonal areas, which with a power 
of 10 diameters are about a third of a line broad. This prismatic 
structure, when present at all in shells, forms the external portion, and 
is considered by Carpenter as corresponding to a calcified epithelium. 
