170 SURVEY OF CELL-LIFE. 



nate throughout its entire thickness. Non-vascular tissues, 

 on the contrary, such as the epidermis, receive the nutri- 

 tive fluid onlv 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 sub- 

 stance. So also in the earlier period of the growth of carti- 

 lage, while it is yet without vessels new cartilage-cells are 

 formed around its surface only, or at least in the neigh- 

 bourhood 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 nutri- 

 tive 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 intussuscep- 

 tion ; (t 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 thick- 

 ness of the organized tissues. The cells, however, grow in- 

 dividually by intussusception in both instances. The bones oc- 

 cupy, 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 



