N. ROSEN, STUDIES ON THE PLECTOQNATHS. 3 



sence of some cartilaginous cells bas been overlooked. In a 

 larva 5 mm. in length the vertebral column is mach more 

 developed. Vertebrae are distinct, but they bave not yet 

 reached tbe definite shape. They are arranged quite meta- 

 merically. Tbe myosepts are inserted between the vertebrae. 

 From this it is evident in what mode tbe vertebra is formed. 

 One vertebra must be the result of a coalescence of the tivo ori- 

 ginal arches of the same metamere of each side and an ossifi- 

 cation of the perichordal tissue of the two metameres in question. 

 Whether the ossification which gives rise to the greater part 

 of the centrum of the vertebra, takes place from two points 

 or not, that is to say whether the centrum bas an diplo- 

 spondylous origin or not, my preparations do not allow me 

 to state. But it is of interest that the neural arch as bas 

 been shown above is formed in a mode that bas not hitherto 

 been proved with certainty to occur in any teleosts. The 

 formation of the vertebrae of Mola is similar to that of Aci- 

 penser. The only difference is the tissue, in Acipenser carti- 

 lage, in Mola bone. The problem of the metamerism of the 

 teleostean vertebrae is not yet settled. Schauinsland con- 

 siders it to be almost evidently proved that in several te- 

 leosts the vertebrae is formed by the coalescence of a caudal 

 portion of one metamere with a cranial one of the foUowing. 

 He is of the opinion, however, that this is not the case in 

 all forms of teleosts. The structure of the vertebrae in some 

 species indicates that the development bas probably been 

 quite a different one, viz. a coalescence of skeleton-portions 

 belonging to the same metamere of each side. It has been 

 shown above that such a mode of development is to be 

 found in Mola. 



In the larva 5 mm. in length the chorda shows some 

 differences in structure to that one in the younger stage (Pl. 

 1. Fig. 2). The chorda is somewhat enlarged intervertebrally. 

 The cells of the epitheliomorfous layer are in these parts very 

 large, almost cyhndrical. The vacuoles are unequal, being 

 compressed intravertebrally. 



Balistidae. 



HoLLARD (3) described the vertebral column quite cor- 

 rectly except that of course his terminology is not the same 



