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 substance between the cells which have reached maturity. In this respect the chorda dorsalis resembles cartilage, but differs again from it, in that, as Müller 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 with 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 substance 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 earlier stage the cavities may be proved to be cells, that is, hollow spaces en-
