166 GENERAL RETROSPECT
ral principle for the formation of all organic productions, and
that this principle is the formation of cells, as well as the conclusions which may be drawn from this proposition, may be comprised under the term cell-theory, using it in its more extended
signification, whilst in a more limited sense, by theory of the
cells we understand whatever may be inferred from this proposition with respect to the powers from which these phenomena result.
But though this principle, regarded as the direct result of these more or less complete observations, may be stated to be generally correct, it must not be concealed that there are some exceptions, or at least differences, which as yet remain unexplained. Such, for instance, is the splitting into fibres of the walls of the cells in the interior of the chorda dorsalis of osseous fishes, which was alluded to at page 14. Several observers have also drawn attention to the fibrous structure of the firm substance of some cartilages. In the costal cartilages of old persons for example, these fibres are very distinct. They do not, however, seem to be uniformly diffused throughout the cartilage, but to be scattered merely here and there. I have not observed them at all in newborn children. It appears as if the previously structureless cytoblastema in this instance became split into fibres; I have not, however, investigated the point accurately. Our observations also fail to supply us with any explanation of the formation of the medullary canaliculi in bones, and an analogy between their mode of origin and that of capillary vessels, was merely suggested hypothetically. The formation of bony lamellae around these canaliculi, is also an instance of the cytoblastema assuming a distinct form. But we will return presently to an explanation of this phenomenon that is not altogether improbable. In many glands, as for instance, the kidneys of a young mammalian foetus, the stratum of cells surrounding the cavity of the duct, is enclosed by an exceedingly delicate membrane, which appears to be an elementary structure, and not to be composed of areolar tissue. The origin of this membrane is not at all clear, although we may imagine various ways of reconciling it with the formative process of cells. (These gland-cylinders seem at first to have no free cavity, but to be quite filled with cells. In the kidneys
