96 COALESCENCE OF THE CELL-WALLS ONLY. 
coalesce and exhibit fibres between the cell-cavities. It does 
not appear to possess any vessels. The formation of new cells 
goes on at the extremities, for instance, at the point of the 
tail of the tadpole; it is not, however, limited to the surface, 
but appears to take place between the most recently-formed 
cells, for cytoblasts may be observed in the intercellular sub- 
stance between the cells which have reached maturity. In 
this respect the chorda dorsalis resembles cartilage, but differs 
again from it, in that, as Miller discovered, it undergoes no 
change in boiling water, and also, in that, the nuclei are flat, 
while those of cartilage-cells are round or elliptical. 
If the chorda dorsalis be reckoned in this class, it affords, 
as we have seen, an example of the generation of cells within 
cells. A different signification might, however, be ascribed 
to these young cells within the true cells of the chorda dorsalis, 
for they do not seem to be formed like their parent-cells, 
from cytoblasts. A generation of cells within cells takes 
place also in the lens. In all the other tissues of this class, 
with few exceptions, the formation of new cells takes place 
only on the outside of those already existing. 
CLASS III. 
Tissues, in which the cell-walls have coalesced with each other, 
or with the intercellular substance. 
This class comprises the firmest structures of the animal 
body, namely, cartilage, bone, and the ivory and osseous sub- 
stance of the teeth. The following is the type of these tissues 
in their mature state: they present either a multitude of small 
roundish cavities in a firm transparent substance, or cavities, 
from which canaliculi issue out in a stellate form; or again, 
merely canaliculi dispersed through the: tissue with tolerable 
regularity. The cavities do not communicate immediately 
with each other; the canaliculi, however, often unite together. 
A special cell-membrane cannot be distinguished in any of 
them in the mature condition, but in an earler stage the 
cavities may be proved to be cells, that is, hollow spaces en- 
