16 



VERTEBRAL COLUMX. 



margins of the groove. At the same time there is enclosed within the cavity the 

 medullary substance derived from the upper layer, which constitutes the rudiment of 

 the brain and spinal marrow. 



It is below and around this tubular cerebro-spinal cavity, and in the substance of 

 the portion of the middle germinal layer immediately adjacent to it, that the cranio- 

 vertebral part of the skeleton takes its origin by steps of which the following is the 

 briefest possible outline. 



Immediately below the cerebro-spinal groove, previous to its closure, there appears 

 in the substance of the middle layer a linear condensed structure, termed chorda 

 dorsalis or notochord ; round this at a later period the base of the skull and bodies, 

 of the vertebra? are developed. The chorda dorsalis may be seen in the embryo-chick 

 after eighteen hours of incubation, and at corresponding periods of advancement in 

 the embryoes of mammalia. In the human embryo, although the first origin of this 

 chord has not yet been observed, there is no reason to doubt its existence ; and from 

 observations at a later period of development it is certain that the foundation of the 

 human skeleton is laid in a manner essentially the same as in animals. 



Soon after the extension of the two upper germinal layers in the dorsal direction 

 has given rise, in the manner mentioned, to the neural cavity and its contained 

 cerebro-spinal axis, the incurvation of all the three germinal layers in a downward or 

 ventral direction results in the formation of the walls of the visceral cavities of the 

 body; and thus from the chorda dorsalis as a centre there proceeds upwards a neural 

 and downwards a visceral arch. 



In the middle layer itself, which is destined for the production of the most com- 

 plicated parts, the thickened portion which is next to the chorda dorsalis on each 

 side is separated from the part which is more remote, and it is in the inner portion 

 now mentioned that there are produced at a very early period those structures which 

 have been called primitive or primordial vertebrae, by embryologists, the formation of 

 which constitutes the next stage in the progress of the earliest development of the 

 skeleton. These rudimental structures consist of small dark quadrilateral patches 

 of condensed germinal substance, to the number of three or four, situated on each 

 Fide of the chorda dorsalis. The anterior of them corresponds to the atlas vertebra, 

 and in the subsequent increase of their number, the new ones make their appearance 

 farther and farther backwards, or towards the caudal extremity. 



Fig. 13. This and the three following figures are intended to 

 illustrate the first formation of the vertebral segments, or "primi- 

 tive vertebrae," and some of the changes by which the permanent 

 vertebral and costal arches are produced. F'g. 13 gives in outline 

 a magnified view of the embryo of the chick about twenty-six hours 

 after the commencement of incubation, as it lies prone in the centre 

 of the germinal membrane. 1, 2, 3, cerebro-spinal canal ; 1 2, 

 the cerebral part ; 2 3, the spinal part, not yet closed in the 

 caudal region ; 4 4, seven primitive vertebrae, proto- vertebras, or 

 vertebral segments ; near 2, the rudiments of the heart. 



The so-called primordial vertebrae have, however, a more ex- 

 tended destination than to serve as the basis of formation of the 

 elements of the vertebral column for each pair of them contains 

 the rudiments, not only of the osseous and cartilaginous parts 

 of a vertebra which cross the middle plane, but also in the 

 thoracic region a portion of a rib, and throughout the whole 

 extent the central parts of a spinal pair of nerves and the 

 cutaneous and muscular parts which cover the whole vertebral 

 region. The process by which these several parts come to be 

 distinct in the progress of development is somewhat intricate. It 

 will be sufficient to mention here that, after a considerable change 

 has occurred in each of the primordial vertebral masses by the 

 rapid formation of cells within them, each becomes cleft into an 

 upper and lower (dorsal and ventral) division or plate. The upper constitutes the 

 basis of the skin and muscles of the vertebral part of the trunk, while the lower or 

 deeper portion undergoes further division in connection with the development of very 



