ONTOGENY AND PHYLOGENY OF THE STERNUM 53 



According to the researches I have made on the development of the 

 sternum in mammals, birds, and batrachians, this bone (sternum) may- 

 be formed in a two-fold fashion. In mammals and birds it occurs 

 under the form of two very slender, long rods (italics mine), divided 

 into two lateral halves, and already at an early period consisting of 

 cartilaginous tissue, each of which rods unites itself with the extremities 

 of several ribs of its own side (italics mine) when these project them- 

 selves through a small part of the lateral wall of the body. The two 

 halves, therefore, at first, lie at a considerable distance from each 

 other. Gradually, however, these two rods are approximated to one 

 another by the extension and development of the ribs, until, at length, 

 they come into contact throughout their whole length, and ultimately 

 coalesce, forming the sternum. 



As regards the Batrachia, even in those which possess ribs, there is 

 never at any time two rods which unite the ribs and coalesce with one 

 another to form the sternum, but in some of these Amphibia there 

 originates a single cartilaginous lamina; in others a row of two or three 

 such laminae quite independent of the lateral rays of the vertebral 

 column 



Rathke's account of the origin of the sternum in birds and 

 mammals gives us a description of a stage far earlier than Ruge's 

 youngest sternum. Ruge saw these 'rods, long and slender' 

 only after they had been united with ribs, and therefrom made 

 his deductions that they originated from ribs. Over thirty 

 years before Ruge's paper, Rathke accurately described this 

 very early stage, which had only been rediscovered in a few 

 mammals very recently. Ruge mentions Rathke's work, but, 

 strange to say, makes no reference to this earlier stage in his 

 account. 



This earlier account by Rathke is hidden in a discussion of 

 the homologies of the Chelonian plastron, and has not been 

 mentioned by any writer since Ruge. Its confirmation by Pat- 

 erson, Whitehead and Waddell, as well as by my own obser- 

 vations, renders Ruge's theory of the sternum untenable. 



However, accurate as this description of Rathke's is, we must 

 not forget that he did not recognize its significance; in fact, 

 it is introduced into a paragraph describing and maintaining 

 that the sternum of the mammals is different in origin from that 

 of batrachians, not realizing as we do to-day that in separating 

 it completely in its genesis from the costal cartilages, he made 

 unnecessary a dual theory of sternal origin. 



