449 



more or less, also assumes a ray -like form, and has its free end directed 

 outwards. Thus a fork is produced, whose one prong is more or less 

 thicker than the other." 



Rathke suggests that the accessory ribs of many fish are probably de- 

 veloped in this way, the upper prong becoming articulated with the lower. 



" The two or three anterior subvertebral processes in the tail are, and 

 remain, quite simple, like the similar processes of the cervical and many 

 dorsal vertebrae. The two halves of the others, which arose as separate 

 lateral processes, remain permanently distinct. 



"All the newly commencing inferior processes arise as paired out- 

 growths of the bodies of the vertebrae." 



" The ribs are not less outgrowths (ausstralilungen) of the bodies of the 

 vertebrae than the crura of the arches, as I can say from my investigations 

 on fishes, snakes, lizards, birds, and mammals j even when the bodies of the 

 vertebrae are completely chondrified, the ribs form a connected whole with 

 them, but subsequently they become articulated, and are thereby essentially 

 distinguished from the crura of the vertebral arches. In some animals the 

 articulation takes place and remains close to the bodies ; in others it takes 

 place also close to the bodies, yet, afterwards, a process grows out between 

 the rib and the body, by which the rib is more or less thrust out ; this is 

 the transverse process ; where it occurs, the rib is, in all cases, at first 

 united only with it ; sometimes, however, a process grows out from the 

 rib (the so-called head with its neck), by which it becomes immediately 

 attached to the body of the vertebra itself, so that it is doubly united 

 with the body. In many cases the rib may also become articulated at some 

 distance from the body, and thus break up into rib and transverse pro- 

 cess." 



" As respects the ribs of the higher Vertebrata, together with their trans- 

 verse processes, the development of the snake teaches us, that although 

 they are subsequently seen to be in close connexion with the crura of the 

 vertebral arches, they grow out, not from the base of these arches, but far 

 from them, out of the bodies of the vertebrae themselves ; where they have 

 arisen, however, each lateral half of the body of the vertebra increases in 

 thickness, in such a manner that it acquires an ala, which drives the crus 

 of the vertebral arch and the rib further and further from the axis, until 

 at length it appears as a common trunk for both, and therefore may easily 

 deceive one into supposing, that the rib is given off from the base of the 

 cms of the arch, and is a process from it." 



2. Lacertilia (Rathke, 'Ueber dieEntwick. d. Schildkroten,' 1848, pp. 

 65-67). In the lizards (as in the snakes), the osseous centra of the ver- 

 tebrae appear as rings, which do not so closely embrace the notochord (as 

 they do in fishes and Batrachia), being separated from it by a layer of 

 cartilage. 



3. Chelonia (Rathke, ' Schildkroten/ p. 65-67). In the Chelonia two 

 bony rings arise, the one on the outer surface of the cartilaginous basis 

 of the centrum, the other close to the notochord ; the rings thicken and 

 eventually coalesce ; the ossification of the arches takes place quite inde- 

 pendently of that of the centrum. The notochord takes no essential part 



