ONTOGENY AND PHYLOGENY OF THE STERNUM 69 



has two backward prolongations, or sternal bars. This sternum 

 is very suggestive of several in the reptiles, and embryos of the 

 mammals, wher.e a single median anterior rudiment is continued 

 backward as two rods or bars. This is an adult specimen and 

 therefore the permanent form in this species ; if, however, we com- 

 pare this with several of the reptiles, such as shown in figures 26, 

 27, and 28, and with descriptions by Rathke, Bruch, Paterson, 

 etc., of the early sternum in the mammals, there appears to be 

 more than a mere resemblance — there is genetic relationship and 

 homology. If in Calamites the coracoids were to retreat to a 

 mere process attached to the scapula, leaving only the clavicle 

 and sternum in this region, and the sternal bars of the latter 

 fused together in the midline, leaving a fossa at the upper end of 

 the union as in many reptiles, and the posterior ends were but 

 incompletely fused leaving two small blunt laterally projecting 

 horns, we would have a sternum such as is actually found in 

 Chirotes (fig. 30) and the embryos of mammals. 



Wilder ('03) describes several cartilaginous rudiments found in 

 Necturus and related by him to the sternal apparatus. These 

 are a series of thin cartilages located in the myocommas of the 

 pectoral region. One of them is usually larger than the rest and 

 situated near the posterior part of the overlapping coracoids. 

 This element is identified by Wilder as the homologue of the 

 sternum of the higher Urodeles. 



If Wilder's theory be correct, Necturus presents an exception 

 to the rule established in this paper that the presternum is a 

 derivative of the coracoids, for obviously this element in Necturus 

 could not possibly be derived from that source. My own dis- 

 sections of Necturus, however, do not bear out Wilder's hypoth- 

 esis. My interpretation of these cartilages is, that they are 

 simply chrondifications of the outer part of the connective tissue 

 of the intermuscular septa and have no relation whatever to the 

 formation of the presternum in higher forms. 



They are rather to be looked upon as subcutaneous splints 

 and find their homologues in the inscriptiones tendineae of other 

 forms, and also possibly in the abdominal ribs of Chamaeleo and 

 Polychrus. It is significant to note in this connection that many 



