170 SURVEY OF CELL-LIFE.
nate throughout its entire thickness. Non-vascular tissues, on the contrary, such as the epidermis, receive the nutritive fluid only from the tissue beneath; and new cells therefore originate only on their under surface, that is, at the part where the tissue is in connexion with organized substance. So also in the earlier period of the growth of cartilage, while it is yet without vessels new cartilage-cells are formed around its surface only, or at least in the neighbourhood of it, because the cartilage is connected with the organized substance at that part, and the cytoblastema penetrates from without. We can readily conceive this to be the case, if we assume that a more concentrated cytoblastema is requisite for the formation of new cells than for the growth of those already formed. In the epidermis, for instance, the cytoblastema below must contain a more concentrated nutritive material. When young cells are formed in that situation, the cytoblastema, which penetrates into the upper layers, is less concentrated, and may therefore serve very well for the growth of cells already formed, but not be capable of generating new ones. This constitutes the distinction which was formerly made between a growth by apposition and one by intussusception; "growth by apposition" is a correct term, if it be applied to the generation of new cells, and not to the growth of those already existing, the new cells in the epidermis for example, are formed only on its under surface, and are pushed upwards when other new ones are formed beneath them; but the new cells are generated throughout the entire thickness of the organized tissues. The cells, however, grow individually by intussusception in both instances. The bones occupy, to a certain extent, a middle position between the organized and unorganized tissues. The cartilage in the first instance has no vessels, and the new cells are, therefore, formed in the neighbourhood of the external surface only; at a subsequent period it receives vessels, which traverse the medullary or Haver sian canals, the latter, however, are not sufficiently numerous to allow of the entire tissue becoming equably saturated with the fluid parts of the blood, a process which would be still further impeded by the greater firmness of cartilage and bone. According to the above law, then, the formation of new cytoblastema and new cells may take place partly upon the
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