DEVELOPMENT OF YERTEBILE. 761 



chorda dorsalis. From these lateral elements are formed 

 the bodies and the arches of the vertebrae. In some 

 animals, as the sturgeon, 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 ver- 

 tebral 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 gra- 

 dually 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 enclose the spinal cord. In this primitive con- 

 dition 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 

 inferiorly by a suture. The chorda is now enclosed in a 

 case, formed by the bodies of the vertebrae, but it gra- 

 dually 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 ob- 

 served at the point where the two primitive elements of 

 the vertebrae have united inferiorly. Those vertebrae 

 which do not bear ribs, such as the cervical vertebrae, 

 have generally an additional centre of ossification in the 

 transverse process, which is to be regarded as an abortive 

 rudiment of a rib. In the foetal bird, these additional 



