DEVELOPMENT OF THE SCALES AND BONES OF FISHES. 
673 
of cells is altered, in accordance with that which they have assumed in the contiguous 
portions of the bone. In the deep sulci intervening between the osseous plates, the 
cartilage has assumed the fibrous aspect already noticed, the fibres being arranged in 
more or less regularly divergent radii, of which the canal of the chorda has been the 
converging point. The rows of turgid and isolated cells follow the direction of 
these semi-fibrous lines in the cartilage. The calcareous additions are made to the 
whole surface where the growing centrum is in contact with the cartilage, but owing 
to the radiating distribution of the cartilage-cells, such of them as are incorporated 
with the sides of the plates, 28 g, h, i, are arranged in rows nearly parallel with the 
line of increment, whilst the cells at their peripheral margins are arranged at right 
angles to that line. I have been particular respecting the relative positions of the 
cartilage-cells and the small cavities existing within the calcareous centrum, be- 
cause we thus learn the important relationship which the one series of structures 
bears to the other. 
When a transverse vertical section of the same vertebra is made near one of its 
extremities, so as to cut off a ring of bone from the margin of one of the terminal 
cones, we find a new modification in the arrangement of the cells. As in the central 
portion immediately surrounding the chorda, they are arranged in concentric circles, 
in which the cells exhibit so strong a tendency to coalesce, as to produce in many in- 
stances elongated concentric tubes, alternating with intervening ribs of calcareous 
matter. By carefully noting the gradual transition from these tubular appearances 
to those portions of the structure where the cavities continue to be isolated and sphe- 
rical, it is easy to see that the differences are merely the result of a modified arrange- 
ment of similar cells. On examining a thin superficial section, taken from the sur- 
face of one of the growing plates, 28 h and /, we perceive that the first deposition of 
the earthy matter takes place immediately around the inflated cartilage-cell, each one 
being surrounded by a granular calcareous fringe. These granules soon coalesce ; 
and the irregular rings thus formed continue to receive additions to their exterior 
until their more salient and contiguous points coalesce. Small angular interspaces 
exist for a while, but in process of time these also become filled up ; and thus the 
cells are reduced to the condition of mere cavities in a solid calcareous structure, in 
which latter, by a careful management of the light, numerous small concentric 
rings may be traced, surrounding the permanent cavities and marking the lines of 
growth. 
These centra contain none of the cancelli or Haversian canals seen in the ordinary 
forms of bone ; the conditions which lead to the formation of both the one and the 
other have obviously no existence here. Bearing in mind the physiological relation- 
ship of these tissues in the Ray to those of the higher mammals, this deficiency of the 
higher osseous elements will be readily understood. Another important fact also to 
be remembered is, that the small circular cells existing in these bones and dermal 
plates, are not the homologues of the canaliculated lacunae, known as the corpuscles 
