. Quain's elements of anatomy . mesoblastic plates, so as to giverise in them to the costal and inter-costal divisions of the Avails. The protovertebral column is atfirst solid, being composed of firmercolumnar cells outside, and loosercellular elements internally. Soon,however, the somites become hollowby the liquefaction of the internalcells ; and it is held that their cavitymay be regarded as an extension of the general body cavity, or theintermesoblastic space, which is so apparent and constant in the lateralplates of themesoblast (Balfour). The mesoblastic somites by their further changes
. Quain's elements of anatomy . mesoblastic plates, so as to giverise in them to the costal and inter-costal divisions of the Avails. The protovertebral column is atfirst solid, being composed of firmercolumnar cells outside, and loosercellular elements internally. Soon,however, the somites become hollowby the liquefaction of the internalcells ; and it is held that their cavitymay be regarded as an extension of the general body cavity, or theintermesoblastic space, which is so apparent and constant in the lateralplates of themesoblast (Balfour). The mesoblastic somites by their further changes give rise to themain constituents of the body wall or framework, that is, the rudimentsof the permanent vertebrae, and the muscular and dermal plates. Thesechanges consist mainly at first in the separation of a mesial or internalpart, in which the vertebral matrices are formed, from the outer partswhich are converted into the muscular and dermal plates in a mannerwhich will be more fully described under their respective II. FORMATION OF THE TRUNK. 1. Vertebral Column.—It is from the inner part of the protovertebralcolumn before mentioned that the blastema of the permanent vertebraeis derived, the rapidly growing cells of which extending inwards andupwards from the protovertebral column, surround the whole of thechorda and the medullary canal with the first, or, as it has been called. YEETEBEAL COLUMN. 795 the membranous matrix of the vertebrse preTious to their cartilaginousdifferentiation. The part of this substance which surrounds the notochord forms atfirst a uniform or undivided tubular enclosure of the chord ; but sub-sequently it shows transverse divisions corresponding to those of theprotovertebral somites. The matrices of the arches, Avhich are con-tinuous with those out of which the bodies are formed, extend dorsallyround the medullary canal and between it and the superficial epiblast,so as gradually to enclose the neural canal. These rudimental neuralarches
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