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 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 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; “ 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 mstance 
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 he 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 
