ote here,
they converge from all directions upon the point wounded or irritated.
They appear to be the active agents in all processes of absorption
(see osteoclasts under bone), and for instance, migrate into and
devour the tissue of the tadpole's tail, during its metamorphosis to the
adult frog.

Section 67. Within the connective tissue cells fat drops may be
formed, as in Figure XV. Adipose tissue is simply connective tissue
loaded with fat-distended cells. The tissue is, of course, a store form
of hydro-carbon (Section 17) provided against the possible
misadventure of starvation. With the exception of some hybernating
animals, such store forms would seem to be of accidental importance
only among animals, whereas among plants they are of invariable and
necessary occurrence.

Section 68. We now come to Bone, a tissue confined to the
vertebrata, and typically shown only in the higher types. As we
descend in the scale from birds and mammals to lizards, amphibia
(frogs and toads) and fish, we find cartilage continually more
important, and the bony constituent of the skeleton correspondingly
less so. In such a type as the dog-fish, the skeleton is entirely
cartilaginous, bone only occurs in connection with the animal's
scales; it must have been in connection with scales that bone first
appeared in the vertebrate sub-kingdom. In the frog we have a
cartilaginous skeleton overlaid by numerous bony scutes (shield-like
plates) which, when the student comes to study that type, he will
perceive are equivalent to the bony parts of such scales as occur in
the dog-fish, sunk inward, and plating over the cartilage; and in the
frog the cartilage also is itself, in a few places, replaced by bony
tissue. In the adult rabbit these two kinds of bone, the bone overlying
what was originally cartilage (membrane bone), and the bone
replacing the cartilage (cartilage bone) have, between them,
practically superseded the cartilage altogether. The structure of the
most characteristic kind of bone will be understood by reference to
Figure XVI. It is a simplified diagram of the transverse section of
such a bone as the thigh bone. M.C. is the central marrow cavity,
H.v., H.v. are cross sections of small bloodvessels, the Haversian
vessels running more or less longitudinally through, the bone in
canals, the Haversian canals. Arranged round these vessels are
circles of the formative elements, the bone corpuscles or
osteoblasts (b.c.) each embe