402 JIIBS. 



(424) H. Leboucq. "Recherch.es s. 1. mode de disparition de la corde dorsnle 

 chez les vertebres superieurs." Archives de Blulogie, Vol. i. 1880. 



(425) Fr. Leydig. Anatomisch-higtologische Untersuchungen uber Fische und Rep- 



tilien. Berlin, 1853. 



(426) Aug. Miiller. "Beobachtungeu zur vergleichendeu Aiiatomie der Wir- 

 belsatile." Miiller' s Archie. 18.33. 



(427) J. Miiller. " V ergleicheude Anatomic der Myxinoideu u. der Cyklostomeu 

 mit durchbohrtem Gaiuneu, I. Osteologie und Myologie." AbhinuUinujrn der ki">i/i/i- 

 llchen Akitdemie der Wisscnschaften zu Berlin. 183-4. 



(428) W. Miiller. "Beobachtungeu des pathologischeu lustituts zu Jeua, I. 

 Ueber den Bau der Chorda dorsalis." Je>iniche Zcitxchrift, Bd. vi. 1871. 



(429) A. Schneider. Beitiaye z. verylcich. Anat. u. Entwick. d. Wirbelthicrc. 

 Berlin, 187'J. 



L'ibs and Sternum. 



Ribs. Embiyological evidence on the development of the ribs, 

 though somewhat inadequate, indicates that they arise as cartila- 

 ginous bars in the connective tissue of the intermuscular septa, 

 and that they are placed (in the Ichthyopsida at any rate) on 

 the level of division between the dorso-lateral and ventro-lateral 

 divisions of the muscle-plates. They may be attached either to the 

 haemal (Pisces) or neural (Amphibia and Amniota) arches. The con- 

 nective tissue from which they are formed is continuous with the 

 processes of the vertebrae to which they are attached ; but the con- 

 version of the tissue into cartilage takes place more or less indepen- 

 dently of that of the arches, although in many cases the cartilage of 

 the two becomes continuous, the separation of the ribs being then 

 effected by a subsequent process of segmentation. (Fick, No. 431). It 

 is possible that the ribs of Pisces may not be homologous with those 

 of Amphibia and the Amniota, but till the reverse can be proved it is 

 more convenient to assume that the ribs are homologous structures 

 throughout the vertebrate series. 



In Elasmobranchii the ribs are relatively of less importance in the 

 adult thau in the embryo. By a careful examination of their early develop- 

 ment, I have satisfied myself that the differentiation of the ribs is indepen- 

 dent of that of the haemal processes to which they are attached, although 

 the differentiation proceeds in such a manner that, when both are converted 

 into cartilage, they are quite continuous. Subsequently the ribs become 

 segmented off from the haemal processes. At the junction of the tail and 

 trunk, where the haemal processes commence to be veiitrally prolonged, 

 eventually to unite in the region of the tail below the caudal vein, the ribs 

 are attached to short processes which spring from the sides of the haemal 

 arches (tig. 317). The ventral haemal arches of these fishes are therefore 

 clearly in no part formed by the ribs. 



In Ganoidei and Teleostei there is very great difficulty in determining 

 the honiologies of the ribs. 



In the cartilaginous Ganoidei there are well developed rib-like struc- 

 tures, which might be regarded as homologous with Elasmobranch ribs, 

 and indeed probably are so ; but at the same time their relations are in 

 some respects very different from those of Elasmobranch ribs in the caudal 

 region. In Ganoids the ribs, in approaching the tail, become shorter and then 

 fuse with the ends of the haemal processes, and finally in the caudal region 



