STRUCTURE AND DEVELOPMENT OF BONE. 
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separated the one from the other, and are no longer to be recognized as distinct 
parts. If accidentally broken across, we see that they have a hollow centre, in fact, 
a lacuna ; but when loose and entire, they appear as rounded dense masses, project- 
ing from the surface of which we may not unfrequently detect a few short needle- 
like processes. Lacunal cells of the latter kind are not, however, very easily de- 
tached, for when they have advanced thus far in development, their union with ossified 
intercolumnar tissue is too strong to admit of ready separation ; indeed, after this 
period their separation is impossible. It has been stated that the granular cells of 
cartilage previous to the formation of lacunal cells send out processes. There are, 
however, a certain number of granular cells that do not appear to undergo this change, 
but preserve their state of granulation and their external figure, and show the round 
nucleus even after they are surrounded by and imbedded in ossified tissue. Indeed, 
we may find perfectly ossified crypts of intercolumnar tissue at some little depth 
within the forming bone, containing several of these granular cells in an unaltered 
condition, and this without the cavities having been enlarged by absorption of their 
walls (Plate VIII. fig. 19 d). It is important to bear this point in mind, as it will sub- 
sequently be shown that similar granular cells lie loose in the large cavities formed 
by absorption, and that they are concerned in the formation of bone when that tissue 
is not preceded by ordinary cartilage. 
If a section of developing bone, including a little of the cartilage, be dried, and 
mounted in Canada balsam, we shall be able to recognize, first the cartilage with the 
cells greatly contracted, then the ossified intercolumnar tissue full of little crypts, from 
which, in the process of drying, the lacunal cells have disappeared (Plate VIII. fig.24 a ) ; 
and lastly, we shall see bone in which ossified lacunal cells run in lines correspond- 
ing to the preceding lines of cartilage cells (fig. 24&). The latter appearance is, 
however, interrupted by the presence of numerous large spaces produced by the 
absorption of both the intercolumnar tissue and the lacunal cells (fig. 24 c), the 
process for the removal of which commences concurrently with the impregnation of 
the lacunal ceils with the earthy salt. 
These conditions can be also very favourably observed in sections of the long 
bones of a foetal lamb, taken previous to the commencement of ossification of the 
epiphysis, and examined in the liquor amnii ; water will not answer as a substitute 
for this fluid, as the cartilage cells are speedily altered in character by its presence. 
And we may here observe that the examination of developing bone should be con- 
ducted on perfectly fresh subjects, and that care should be taken to keep the knives, 
glass, &c., which are used in preparing the sections, perfectly free from extraneous 
matter; otherwise the results will be unsatisfactory. Thus a small quantity of spirits 
of wine or of acid will produce changes of character in the specimen, and thereby 
lead the observer into an erroneous conception of the normal characters of the tissue. 
Hitherto the authors of this communication have spoken principally of those ap- 
pearances which are presented in sections made parallel with the advancing bone ; 
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