UATHKE ON TIIE DEVELOPMENT OE TnE CEANIHM. 237 
responding to the costal cartilage, and receiving the name of a 
sternal rib. 
§. 7. What has been stated respecting the development of the 
vertebral column relates only to the ordinary process. From this, 
however, there are many and sometimes important departures. And 
inasmuch as these may be of importance in making a comparison 
between the spinal column and the skull, I shall here give a brief 
account of the chief of them. 
§. 8. In the Myxinoids and in Ammoccetes branchialis, the invest¬ 
ing mass, which immediately surrounds the notochord, becomes every 
where metamorphosed into a moderately thick fibrous membrane, 
which completely ensheathes the notochord. From the sheath two 
plates of like texture pass upwards to embrace the spinal cord and to 
unite above it. Petromyzon exhibits a somewhat higher develop¬ 
ment. The notochord and spinal marrow are inclosed within a similar 
fibro-membranous investment, but there is, in addition, a slight indica¬ 
tion of proper vertebral bodies, which consists in two very narrow 
and short cartilaginous striae, into which the basal part of the 
occipital cartilage is prolonged posteriorly, and which are situated 
on both sides of the notochord, on the above-mentioned fibrous 
sheath. They may be regarded as indications of many coalesced 
vertebral bodies; and the more so, as on each side, immediately behind 
them, a short longitudinal series of a few very small cartilaginous 
plates appears, representing the separate lateral halves of a corres¬ 
ponding number of vertebral centra. Notwithstanding that only 
these scanty traces of a few vertebral centra appear in Petromyzon , 
cartilaginous vertebral arches are developed in great numbers, em¬ 
bracing the spinal chord and fixed upon the external, almost wholly 
fibro-membranous sheath of the notochord ; being inclosed in the 
substance of the two fibro-membranous plates which pass from that 
sheath upwards, to inclose the spinal chord laterally. 
In the Sturgeon we have a still higher grade of development, 
though not so high as in the Plagiostomes. Not only are the crura 
of vertebral arches present, as in Petromyzon, but far more definite 
rudiments of vertebral centra make their appearance, consisting, (if 
we except the four or five anterior vertebrae) for each vertebra of four 
larger or smaller, more or less distant, cartilaginous plates closely 
united with the fibrous sheath of the notochord ; viz., of two upper 
and two lower ones, so that on each side an upper and a lower series 
of such plates runs along the spinal column. From the upper ones, 
the cartilaginous vertebral arches proceed, and ribs pass as immediate 
prolongations of a few of the lower ones, whilst with others, espe¬ 
cially the most anterior, the ribs are moveably articulated. The four 
or five most anterior centra consist each of a perfectly closed ring, 
which has without doubt arisen from the gradual increase and ulti¬ 
mate coalescence of the four cartilaginous plates. 
The structure of the vertebral column of the Sturgeon shows 
plainly,that in these Fishes, the investing mass of the notochord gives 
