DEVELOPMENT OF ELASMOBRAXCH FISHES. 153 



veins fuse to form an impaired 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 pro- 

 cesses 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 PL xii. fig. 10), 

 a fact which shows (1) 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 processes 

 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\ 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 (PL xii. fig. 11). 

 The innermost layer adjoining the notochord retains its primi- 

 tive fibrous character, and is distinguishable as a separate layer 

 through both the vertebral and the intervertebral regions. As a 

 result of these changes a transverse section through the centre 

 of the vertebral reo^ions now exhibits three successive rings 

 (vide PL XII. fig. 11), an external ring of hyaline cartilage in- 

 vested by ' the membrana elastica externa' {m.el), followed by a 



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. 1), who 

 also gives a fair description of the succeeding changes of the vertebral column. 



B. 11 



