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 in 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 to the 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 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 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 lamella 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 



