Cuap. II. FORMATION OF THE ORGANS. 593 
consists of the vertebral column already described, the fibrous corium, (PL 9e, fig. 12, 
c,) and the ribs (a, 6) imbedded in the latter. The ribs do not touch each other 
laterally, as in the adults of many genera, but their margins (Pl. 9e, fig. 12, 5) 
are flattened, and run out into a thin edge. At the surface of the ribs, the 
passage is so gradual from the fibrous bony layer (Pl. 22, fig. 1, 4, ¢ d, e f. 
2, h,) that 
it is impossible to distinguish the one from the other. In fact, since the ribs 
fig. 2, 6, ¢, d, e, f) mto the fibrous corium, (fig. 1, gy, g', h, I, fig. 
are developed from what was once a uniform layer of corium throughout the whole 
extent of the shield, it is not at all to be expected that the line of demarca- 
tion should be very definite at this age. The true cartilagino-osseous matrix of 
the ribs is in the interior of each rib (Pl. 9e, fig. 12, a, a; Pl. 22, fig. 1, a, 
Gena, fig. \2,\ a,)-a';~ a7, 0°). : 
The Limbs, The changes through which the limbs pass, as far as their exter- 
nal configuration is concerned, have been sufficiently described in the last section. 
At first they are lateral protrusions (Pl. 24, fig. 14, w’, p. 555) of the musculo- 
cutaneous layer from the sides of the body, and are composed of a solid mass 
of cells, identical with each other throughout the whole limb. The bone, or 
rather the matrix of the bone, is formed by a certain portion of the cells in 
the axis of the limbs becoming differentiated from the others, by increasing in 
size and assuming a less elongated shape, (Pl. 21, fig. 22, 22a, 24,) and at the 
same time separating from each other. This arrangement produces a different effect 
upon the light from that of the cells in the surrounding parts, so that the posi- 
tion of each bone-matrix may be recognized without much trouble (Pl. 25, fig. 11). 
The matrices are not sharply defined, because the passage from the elongated cells 
of the unossifying portion to the broad cartilage cells of the matrix is not sud- 
den, but gradual. Later, however, the bone matrix becomes more definite in 
outline, (Pl. 21, fig. 21; Pl. 25, fig. 12, 4, ¢, d,) and differs very much in appear- 
ance from the surrounding portion of the limb (Pl. 21, fig. 20, 4, 0’; Pl. 25, fig. 
12, a). About the time of hatching, the bones are all well developed, and have 
in a great measure the characteristic shape of those of the adult, but the ossifi- 
cation is only external, the central part of the bones being occupied by almost 
pure cartilage (Pl. 22, fig. 5, 6, 6a, 6b). The bones of the feet are the most 
ossified, especially the terminal ones ; and not only is the fibrous layer hardened, 
but also a considerable thickness of the cartilaginous basis contains lime. The 
femur comes next in amount of ossification, but this occurs only along the cylin- 
der, and not at all on the ends. The scapula, and its process, the acromion, are 
but a little less ossified than the last. The other bones have a very thin exter- 
nal fibrous ossified layer, the rest of each bone being highly cartilaginous. 
The Muscles. During our investigation of the development of the Turtles, our 
75 
