. Quain's elements of anatomy . f tiie bodies ofthe vertebrffi ; e, slight dilatations of the noto-chord opposite the epiphysial jDlates. canal. This takes place on the third day inthe chick, and is therefore considerably laterthan the investment of the notochord with thevertebral elements of the bodies. The membranous investment of the me-dullary canal doubtless contains the elementsnot only of the neural vertebral arches, but alsothose of the dura mater and other coveringsof the central nervous organs. This invest-ment does not however form a complete tube,but is interrupted or open at the i
. Quain's elements of anatomy . f tiie bodies ofthe vertebrffi ; e, slight dilatations of the noto-chord opposite the epiphysial jDlates. canal. This takes place on the third day inthe chick, and is therefore considerably laterthan the investment of the notochord with thevertebral elements of the bodies. The membranous investment of the me-dullary canal doubtless contains the elementsnot only of the neural vertebral arches, but alsothose of the dura mater and other coveringsof the central nervous organs. This invest-ment does not however form a complete tube,but is interrupted or open at the intervalsoccupied by the spinal ganglia and nervesemanating from the medullary centre. Whenchondrification of the vertebral matrices takesplace the neural arches are found to be connected and at first continuous withthe forepart of each vertebral body as reconstituted by the secondary transversedivision, the intervals for the spinal nerves and ganglia being opposite theposterior part of tlje bodies and the intervertebral 70S FEAMEWORK OF THE BODY. The Mstoiy of the further development of the vertebral column belongs ratherto that of the ossification of its several parts, which has been described in thefirst volume. It may be stated in addition here that in the human embryo theprocess of chondrification of the bodies begins between the fouith and fifthweeks (KoUiker), and is already completed by the sixth or seventh, soon afterwhich ossification commences. In the arches the chondrification is about a weeklater in commencing, but the ossific deposit, which begins in the eighth week, isslightly earlier in them than in the bodies. The whole number of cartilaginous vertebral matrices varies from 33 to 35,there being occasional subordinate variations in the number developed in eachdivision of the trunk. The maximum number of coccygeal pieces is six accord-ing to E. Rosenberg (No. 166). The formation of cartilaginous matrices for the vertebral arches begins firstin those of th
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