360 Mr. H. G. Seeley on a Theonj 



surface of that bone, and to enter into the condyle. But they 

 differ from the inferior epiphyses of vertebrae in being united and 

 never surrounding any vessels ; and therefore, perhaps, they are 

 rather to be regarded as distinct ossifications peculiar to the skull. 



As we have already remarked, the mouth is the prehensile end 

 of the digestive canal, and in Amphioxus it is surrounded by jointed 

 rings of cartilage. And, ascending in organization, it were easy 

 to trace, by way of the lampreys and sharks, the gradual union 

 between the jaws and the skull ; and therefore we have to dis- 

 cover the origin and the law which governs the uniformity of 

 development of these bones of the face. 



And here I seek the aid of embryology to resolve the bones 

 into their natural groups, though somewhat reluctantly, because 

 the results from one ti-ibe of animals cannot hold quite true for 

 another tribe where the organization differs ; but it is so conclusive 

 on the significance of the jaws, that I will give, in a translation 

 of Professor Rathke's own words, his remarks on their origin. 

 He says, " That part of the investing mass of the notochord in 

 which the basisphenoid is developed in many animals, sends 

 out a 'ray' or band downwards on each side, which presents 

 a remarkable similarity to a rib, not only in its mode of origin, 

 but in its original position and form." These, then, it will be 

 seen, are the true epiphyses which correspond to ribs, and, as was 

 to be expected, they grow out of the basisphenoid, which was 

 the original centrum of the skull; and as the true ribs grow 

 down to enclose the posterior part of the digestive organ, so we 

 shall see these ribs grow down to embrace its anterior end, and 

 become modified into prehensile organs. Professor Rathke goes 

 on to say, " But very early there grows out from near the 

 upper end of the ray a long thin process, which passes off at 

 an obtuse angle to it and applies itself to the inferior wall of the 

 future brain-case." Thus the ribs, growing down on the diges- 

 tive canal, appear to become split, and the upper parts run along 

 the top of it and the lower parts run down the sides, thus 

 eventually coming to embrace the mouth without bringing it in 

 contact with the centrum ; but it ought to be remembered that, 

 in the adults of all the animals in which this is observed, union 

 has already taken place between the face and the brain-case. 



That the ribs really become split as they apparently do, 

 I do not see any reason for believing, and should rather regard 

 the upper portions of the forks as connate growths produced by 

 causes presently to be considered. The proximal end of the 

 cranial representative of a rib ossifies and becomes the quadrate 

 bone or incus ; an intermediate part becomes the os articulare ; 

 while the distal end remains unossified, but developes bones on 

 its surface which become the lower jaw. 



