CHAP, x.] THE DEVELOPMENT OF THE CAT. 333 



The earliest indication of the segmented condition of the VERTEBRAL 

 SKELETON is the appearance of the quadrate masses of tissue, appear- 

 ing serially in pairs behind the head the so-called proto-vertebrcB 

 (Fig. 146, pv), or dorsal segments, already noticed as arising in the 

 mesoblast on each side of the chorda dorsalis and medullary groove. 

 The front part of the first of these bodies corresponds in position with 

 the atlas, and each pair of dorsal segments gives rise, amongst other 

 structures, to part of the bony spine. The dorsal segments do not, 

 however, correspond with the future vertebra, but each segment 

 becomes transformed into : (1) the hinder part of one vertebra, (2) 

 the anterior part of the vertebra next behind, (3) the roots of a 

 spinal nerve, and (4) the muscles and skin immediately connected 

 with the vertebral parts so formed. Thus each primordial vertebra 

 becomes ultimately segmented, while each successive pair of such 

 primordial vertebrae ultimately coalesce, and so a different segmenta- 

 tion is brought about from that which appears at first ; the points of 

 separation of the later segmentation, alternating with those of the 

 earlier segmentation. The first dorsal segment of each side differs 

 from those which succeed, in that when it becomes segmented, its 

 anterior half has nothing in front wherewith to coalesce, and thus 

 the atlas must be formed from half a primordial vertebra instead 

 of being formed, like the other vertebra, from the hinder half of one 

 primordial vertebra and the anterior half of the primordial vertebra 

 next behind. 



Moreover, while the upper part of each dorsal plate becomes a 

 segment of dorsal muscle, with its skin, the lower part undergoes a 

 different change in its anterior (cephalic) and posterior halves. An- 

 teriorly it gives rise to the root and ganglion of a spinal nerve. 

 Posteriorly it gives rise to the transverse process of a vertebra or 

 proximal portion of a rib. 



The inner part of each dorsal plate bifurcates as it extends 

 inwards. One branch ascends in the dorsal lamina till it meets its 

 fellow of the opposite side and so forms the foundation of the future 

 neural arch. The other branch advances inwards above and below 

 the chorda, and blends with its fellow of the opposite side to form 

 the foundation of the central portion of a vertebral segment. Thus 

 each permanent vertebra is the offspring of the parts of two adjacent 

 primordial vertebra. Its neural arch, transverse processes, the 

 proximal part of its ribs, and the cephalic portion of its centrum, 

 are formed from the hinder end of one proto-vertebra, while the 

 rest of its centrum and its spinal nerves are formed from the anterior 

 (cephalic) end of the proto-vertebra next behind. 



This condition having been, as it were, laid down in soft tissue, 

 transformations of parts of the structures thus formed, into cartilage, 

 soon begin. 



Cartilage of the ordinary kind first invades the body of ^ each 

 vertebra and surrounds the chorda, encroaching on and constricting 

 it at intervals, the chorda yet continuing a structure of large thin- 

 walled cells enclosed in a fibrous sheath. Cartilage becomes also 



