ONTOGENY AND PHYLOGENY OF THE STERNUM 75 



sternum kept on growing until the respective pairs met and fused 

 in the midventral line. Chamaeleo is considered to present 

 strong evidence of the secondary character of the relation between 

 ribs and sternum, and it is an important intermediate stage in 

 the development between the typical reptilian sternum and the 

 same structure in the mammals. 



The preceding account of the development and anatomy of 

 the sternum in the Reptilia and a comparison with the same struc- 

 ture in the Amphibia must inevitably lead to the conclusion 

 that, if the presternum be coracoidal in origin in the Amphibia, 

 it is equally so in the Reptilia. For, beginning with that most 

 primitive reptilian sternum in Anguis, and comparing with Pipa 

 and other Amphibia, the gulf was bridged between these two 

 phyla, and then by a series of successively more highly developed 

 sterna in the reptiles a stage is reached (Chirotes) which spans 

 the divide between the reptiles and mammals. We have also 

 seen how in one amphibian (fig. 24) the beginning of sternal 

 bands arises, and in the reptiles these are developed in the same 

 way, and in higher reptiles fuse to form the mesosternum and, 

 xiphisternum, preparing the way for the typically mammalian 

 sternum, soon to be considered. 



The last fact concerning the Reptilia is in regard to the ex- 

 tremely variable relation of ribs and sternum, both as to number 

 and position. Rathke ('53) had an interesting paragraph on this 

 which is quoted in part : 



In typical scaly lizards several ribs are always in relation with the 

 sternum; still .... it may be either only the anterior divi- 

 sion (manubrium) which is connected with ribs, or it may be exclusively 

 the posterior part. But, generally speaking, the number of ribs which 

 are intimately connected with the sternum, and to which the name of 

 true ribs can be applied, not only varies with the genus, but is also 

 very variable in different species. 



Both Rathke and Parker give long lists of species of reptiles 

 showing this variability in the number and position of ribs 

 reaching the sternum. Their lists comprise some fifty species, 

 but only a few are mentioned here as indicating the range of 

 variation found by them. Their figures show that in some species 



