594 GENERATION AND DEVELOPMENT. 



Development of the Vertebral Column and Cranium. 



The primitive part of the vertebral column in all the Ver- 

 tebrata is the gelatinous chorda dorsalis, which consists entirely 

 of cells. This cord tapers to a point at the cranial and caudal 

 extremities of the animal. In the progress of its development, 

 it is found to become inclosed in a membranous sheath, which 

 at length acquires a fibrous structure, composed of transverse 

 annular fibres. The chorda dorsalis is to be regarded as the 

 azygos axis of the spinal column, and, in particular, of the 

 future bodies of the vertebrae, although it never itself passes 

 into the cartilaginous or osseous state, but remains inclosed as 

 in a case within the persistent parts of the vertebral column 

 which are developed around it. It is permanent, however, 

 only in a few animals: in the majority it disappears at an 

 early period. 



The cartilaginous or osseous vertebrae are always first de- 

 veloped in pairs of lateral elements at the sides of the chorda 

 dorsalis. From these lateral elements are formed the bodies 

 and the arches of the vertebrae. In some animals, as the stur- 

 geon, however, the lateral elements of the vertebrae undergo 

 no further development, and it is here that the chorda dorsalis 

 is persistent through life. In the myxinoid fishes the spinal 

 column presents no vertebral segments, and there exists merely 

 the chorda dorsalis with the fibrous layer surrounding its 

 sheath, which is the layer in which the skeleton originates. 

 This fibrous layer also forms superiorly the membranous cover- 

 ing of the vertebral canal. 



In reptiles, birds, and mammals, the mode in which the 

 vertebrae are formed around the chorda dorsalis seems to be 

 different. When the formation of these parts from the blastema 

 commences, there appears at each side of the chorda dorsalis 

 a series of quadrangular figures, the rudiments of the future 

 vertebrae. These gradually increase in number and size, so 

 as to surround the chorda both above and below, sending out, 

 at the same time, superiorly, processes to form the arches des- 

 tined to inclose the spinal cord. In this primitive condition 

 the body and arches of each vertebra are formed by one piece 

 on each side. At a certain period these two primary elements, 

 which have become cartilaginous, unite iuferiorly by a suture. 

 The chorda is now inclosed in a case, formed by the bodies of 

 the vertebrae, but it gradually wastes and disappears. Before 

 the disappearance of the chorda, the ossification of the bodies 

 and arches of the vertebrae begins at distinct points. 



The ossification of the body of a vertebra is first observed 

 at the point where the two primitive elements of the vertebrae 



