STRUCTURE AND DEVELOPMENT OF LEPTDOSTEUS. 785 



those two parts at this stage are not due to a secondary fusion, 

 but to a commencing segmentation. 



It further appears, as Mtiller, Gegenbaur and others have 

 stated, that the ribs and haemal processes of the tail are serially 

 homologous structures ; but that the view maintained by Gotte 

 in his very valuable memoirs on the Vertebrate skeleton is also 

 correct to the effect that the Jicemal arches of tlie tail are Jwmo- 

 logous throughout the series of FisJies. 



To this subject we shall return again at the end of the 

 section. 



Before leaving the haemal arches it may be mentioned that 

 behind the region of the ventral caudal fin the two haemal pro- 

 cesses merge into one, and form an unpaired knob resting 

 on the ventral side of the notochord, and not perforated by 

 a canal. 



There are now present well -developed intervertebral rings of 

 cartilage, each of which eventually becomes divided into two 

 parts, and converted into the adjacent faces of the contiguous 

 vertebrae. These rings are united with the neural and haemal 

 arches of the vertebrae in front and behind. 



Each ring, as shewn by the transverse section (Plate 41, fig. 

 71), is not uniformly thick, but exhibits four projections, two 

 dorsal and two ventral. These four projections are continuous 

 with the bases of the neural and haemal arches of the adjacent 

 vertebrae, and afford presumptive evidence of the derivation of 

 the intervertebral rings from the neural and haemal arches; in 

 that had they so originated, it would be natural to anticipate 

 the presence of four thickenings indicating the four points from 

 which the cartilage had spread, while if the rings had originated 

 independently, it would not be easy to give any explanation of 

 the presence of such thickenings. Gegenbaur (No. 6), from the 

 investigation of a much older larva than that we are now describ- 

 ing, also arrived at the conclusion that the intervertebral carti- 

 lages were derived from the neural and haemal arches ; but as 

 doubts have been thrown upon this conclusion by Gotte, and 

 as it obviously required further confirmation, we have considered 

 it important to attempt to settle this point. From the description 

 given above, it is clear that we have not. however, been able 

 absolutely to trace the origin of this cartilage, but at the same 



