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 conclu- 
sions which may be drawn from this proposition, may be com- 
prised under the term cell-theory, using it m its more extended 
signification, whilst in a more limited sense, by theory of the 
cells we understand whatever may be inferred from this pro- 
position with respect tothe powers from which these pheno- 
mena 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 unex- 
plained. Such, for instance, is the splitting mto 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 carti- 
lage, but to be scattered merely here and there. I have not ob- 
served them at all in new-born 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 lamelle 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 fcetus, 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 
