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 Miiller 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 asrain, 

 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 earlier stage the 

 cavities may be proved to be cells, that is, hollow spaces en- 



