562 STERNUM. 



to trace the ribs bending ventral-wards, and uniting ventrally in the caudal 

 region to form, with the haemal processes, a complete haemal canal. 



It might have been anticipated that the Teleostean Ganoids would 

 resemble the Teleostei, but, from an examination of adult Teleostei, it 

 would seem to be clear that the relations of the parts are the same as in 

 Elasmobranchii, i.e. that the ribs have no share in forming the haemal 

 canal in the tail. Aug. Miiller and Gotte have however brought embryo- 

 logical evidence (though not of a conclusive character), to shew that in the 

 embryo the ribs really fuse with the haemal processes in the tail, and so 

 assist, as in the Ganoids, in forming the haemal canal. Gotte moreover holds 

 that the ribs in Elasmobranchii are not homologous with those of Teleostei 

 and Ganoids ; but that the haemal arches in the tail are homologous in the 

 three groups. 



Without necessarily following Gotte in these views it is worth pointing 

 out that the undoubtedly close affinity between the bony Ganoids and the 

 Teleostei is in favour of the view on the haemal arches of Teleostei at 

 which he has arrived on embryological grounds. 



In Amphibia the formation of the ribs from the connective tissue of the 

 intermuscular septa, their secondary attachment to the transverse processes 

 of the neural arches, and their subsequent separation was first clearly 

 established by Fick (No. 431), whose statements have since been confirmed 

 by Hasse, Born, &c., and in part by Gotte, who holds however that, though 

 converted into cartilage independently of the transverse processes, they 

 are formed in membrane as outgrowths of these processes. 



In the Amniota the ribs are also independently established (Hasse and 

 Born), though they subsequently become united to the transverse processes 

 and to the bodies of the vertebrae, or to the transverse processes only. 

 This junction is however stated by the majority of authorities, never to 

 be effected by the fusion of the cartilage of the two parts, but always by 

 fibrous tissue ; though Hoffmann (No. 435) takes a different view on this 

 subject, holding that the ribs are at first continuous with the intervertebral 

 regions of the primitive cartilaginous tube surrounding the notochord. 



Sternum. In dealing with the development of the sternum 

 it will be convenient to leave out of consideration the inter- 

 clavicle or episternum which is, properly speaking, only part of 

 the shoulder-girdle and to confine my statements to the sternum 

 proper. 



This structure is found in all the Amniota except the 

 Ophidia, Chelonia, and some of the Amphisbaenae. 



From the older researches of Rathke, and from the newer 

 ones of Gotte, etc., it appears that the sternum is always formed 

 from the fusion of the ventral extremities of a certain number of 

 ribs. The extremities of the ribs unite with each other from 



