374 DEVELOPMENT OF ELASMOBRANCH FISHES. 



centres of the vertebrae. These processes throughout the region 

 of the trunk in front of the anus pass into the space between the 

 dorsal and ventral muscles, and are to be regarded as rudiments 

 of ribs. The tissue with which they are continuous, which is 

 exactly equivalent to the tissue from which the neural arches 

 originate, is not truly a part of the rib. In the tail, behind the 

 anus and kidneys, the cardinal veins fuse to form an unpaired 

 caudal vein below the aorta, and in this part a fresh series of 

 processes originates on each side from the haemal tissue adjoining 

 the cartilaginous sheath of the notochord, and eventually, by the 

 junction of the processes of the two sides, a canal which contains 

 the aorta and caudal vein is formed below the notochord. These 

 processes for a few segments coexist with small ribs (vide PI. 13, 

 fig. 10), a fact which shews (i) that they cannot be regarded as 

 modified ribs, and (2) that the tissue from which they spring is 

 to be viewed as a kind of general basis for all the haemal pro- 

 cesses which may arise, and is not specially connected with any 

 one set of processes. 



While these changes (all of which are effected during stage 

 P) are taking place in the arches, the tissue of the vertebral 

 bodies or cartilaginous investment of the notochord, though 

 much thicker than before, still remains as a continuous tube 

 whose wall exhibits no segmental differentiations. 



It is in stage Q that these differentiations first appear in the 

 vertebral regions opposite the origin of the neural arches. The 

 outermost part of the cartilage at these points becomes hyaline 

 and almost undistinguishable in structure from the tissue of the 

 arches 1 . These patches of hyaline cartilage grow larger and cause 

 the vertebral parts of the column to constrict the notochord, 

 whilst the intervertebral parts remain more passive, but become 

 composed of cells with very ' little intercellular substance. 

 Coincidently also with these changes, part of the layer internal 

 to the hyaline cartilage becomes modified to form a somewhat 

 peculiar tissue, the intercellular substance of which does not 

 stain, and in which calcification eventually arises (PI. 13, fig. 11). 

 The innermost layer adjoining the notochord retains its primitive 



1 A good representation of a longitudinal section at this stage is given by Cartier 

 (Zeitschrift f. Wiss. Zoologie, Bd. xxv., Supplement PI. iv. fig. i), who also gives a 

 fair description of the succeeding changes of the vertebral column. 



