462 RIBS. 



{424) H. Leboucq. "Kecherchea s. 1. mode de disparition de la coide dorsale 

 chez Itb vertebres superieurs." Archives de Biologle, Vol. i. 1880. 



(425) Fr. Leydig. Anatomisch-histologische Untersuclmnijenuher FLclie unci Rep- 

 tilien. Berlin, 1853. 



(426) Aug. Miiller. " Beobachtungen zur vergleichenden Auatomie der Wir- 

 belsaiile." Mniler's Archiv. 1853. 



(427) J. Miiller. " V ergleichende Anatomie der Myxinoideii u. der Cyklostonieu 

 mit durchbohrtem Gaumeii, I. Osteologie und Myologie." Abhandluiujen der k'mig- 

 lichen Akademie der Wissenschoften zu Berlin. 1834. 



(428) W. Miiller. "Beobachtungen des pathologisclien Instituts zu Jena, I. 

 Ueber den Bau der Chorda dorsalis." Jenaische Zeitschrift, Bd. vi. 1871. 



(429) A. Schneider. Beittage z. vergleich. Anat. u. Entwick. d. Wirbelthiere. 

 Berlin, 1879. 



Bibs and Sternum. 



Bribs. Embryological 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 

 ha?mal (Pisces) or neural (Amphibia and Amniota) arches. The con- 

 nective tissue from which they are formed is continuous with the 

 processes of the vertebrai 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 Elasmobrancliii the ribs are relatively of less impfirtance in the 

 adult than 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 ventrally 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 foi-med by the ribs. 



In Ganoidei and Teleostei there is very great difficulty in determining 

 the homologies 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 



