ONTOGENY AND PHYLOGENY OF THE STERNUM 55 



region are the only ones which can be lined up with the lower 

 groups of vertebrates. The rat was used by Paterson and the 

 mouse is the basis of the present paper, and this would seem to 

 be the ideal form; for the rodents, while highly specialized in 

 some respects, are primitive in others, and are to be grouped with 

 the Edentates and Insectivores somewhere near the monotreme 

 stem. They are also small enough so that when sectioned, com- 

 paratively high powers of the microscope may be used, and it is 

 possible to section all stages up to the ripe fetus. 



Discarding, then, Whitehead and WaddelFs theory of sternal 

 origin, while retaining an appreciative memory for their valuable 

 work in combating one of the remaining theories, we can reduce 

 the great mass of papers and discussion on this subject to just 

 two absolutely irreconcilable theories of sternal origin, which 

 may for convenience in treatment be designated as Ruge's 

 'theory of costal origin,' and Paterson's 'theory of coracoidal 

 origin.' All other workers, except Whitehead and Waddell, have 

 supported one or other of these theories or modifications of 

 them. 1 



1 Through the courtesy of Doctor Kingsley, I have just received two papers 

 on the sternum, one of which requires mention. This is by Albrecht: Sur les 

 Copulae Intercostoidales et les Hemisternoides du Sacrum des Mammifers. 

 Bruxelles, 1883. It contains a most curious modification of Ruge's theory of 

 costal origin. Albrecht's idea is that the first and second ribs of each side at 

 first are united by an arch of cartilage, giving, according to his schematic figures, 

 a structure similar to a horseshoe magnet, the two arms of the magnet being the 

 ribs, the arch connecting them the sternal band. Then by a union of the two 

 sides in the midline, and the fusion of the consecutive pairs of such magnet- 

 shaped structures, a sternum is derived. This is ingenuous and is the only 

 theory of costal origin which gives the sutures between the sternebrae their 

 proper position, i.e., opposite the ends of the ribs, making the sternebrae inter- 

 costal as they actually are. However, the arguments used to overthrow Ruge's 

 theory apply equally here. This theory does not account for the anterior and 

 posterior extension of the sternal bands for a considerable distance beyond the 

 region of ribs; it does not explain the appearance of the bands prior to their 

 union with ribs; and fatal to Albrecht's hypothesis is the fact that the bars are 

 continuous, unsegmented structures throughout their entire length from their 

 earliest appearance in the mesenchyme, and never occur in short, semicircular 

 segments connecting the ends of the ribs. Albrecht was evidently unable to 

 find any stages in actual material in support of his theory, for his figures with- 

 out exception are diagrammatic, and do not fit the observed facts of sternal 

 development. 



