ON THE THEORY OF THE VERTEBRATE SKULL 603 



In man and in the pig, the heads of the ribs, whether dorsal or cervical, are 

 as Retzius has well pointed out) articulated above the neurocentral suture ; 

 and therefore, if we accept the ordinary definition, not to the centrum at all, 

 but to the neurapophysis. Furthermore, if we accept the ordinary view, the 

 " inferior transverse processes " in the neck of these animals are not parapophyses, . 

 but second diapophyses, inasmuch as they arise from the neurapophyses, and not 

 rom the centrum. 



The "transverse processes" of the lumbar vertebrae are usually given off' 

 above the neurocentral suture, and are therefore called "diapophyses." In a young 

 Dugong, in the Museum of the Royal College of Surgeons, I find that, in the two 

 hinder lumbar vertebrae, these transverse processes are given off below the 

 neurocentral suture. 



In the Echidna the head of every rib is attached to the centrum, or below the 

 neurocentral suture ; and in the neck, this suture lies between the upper and lower 

 transverse processes. 



Thus, if we follow out logically the view that the neurocentral suture indicates 

 the boundary between the neurapophyses and the centrum, and if we accept the 

 current definition of diapophyses and parapophyses, we arrive at the conclusion, 

 that, in the cervical region, man and the pig have vertebrae with two diapophyses 

 and no parapophysis, while the monotreme has a parapophysis and a diapophysis 

 on each side ; that, in the dorsal region, the ribs in man and the pig are con- 

 nected only with the neurapophyses, while in the monotreme they articulate 

 only with the centra ; that the transverse processes of the anterior lumbar 

 vertebrse of the dugong are diapophyses, while those of the posterior ones are 

 parapophyses ! 



The crocodile and some extinct reptiles, such as the Iclithyosauriis, whose ribs 

 are throughout attached to the centra, afford still more striking instances of the 

 confusion which would be produced by taking the neurocentral suture as a morpho- 

 logical boundary. 



Miiller has argued that it is a distinctive character of fishes to have the ribs 

 attached to the centre of the vertebrae or to parapophyses, and his views have 

 been adopted by other anatomists ; but the ribs of the Ichthyosaurus and of the 

 Echidna are as completely and solely attached to their centra as those of any fish ; 

 so that if we merely take the facts furnished by anatomy, the doctrine that there is 

 anything peculiarly piscine in the attachment of the ribs to the centra only, falls to 

 the ground. 



But it may be urged that the connexion of the head of the rib with the centrum, 

 in mammals and the higher Vertebrata, is secondary, that with the diapophysis 

 being the primary and essential one. This is a very widely current doctrine, and 

 it is sanctioned by Rathke, as we have seen in the passages quoted above, though 

 they are not quite consistent with one another. It is with great hesitation that I 

 venture to contravene the distinct statements of so eminent and accurate an 

 embryologist as Rathke, but my own observations lead me to precisely the 

 opposite conclusions. 



In the spinal column of embryos of the mouse 7-8ths of an inch long, for 

 instance, I find that the posterior dorsal vertebra; (A. fig. 10) have no diapophyses, 

 and that the ribs have no tubercles, but that their heads pass directly into the 

 cartilaginous substance of the centrum. Further forwards (B, C) both diapophysis 

 and tubercle become more and more developed, until at length they come into 

 contact and articulate in the ordinary way. Finally, in the cervical region (D) the 

 rib and the diapophysis are, even in this early stage, confluent. 



These facts are, I conceive, wholly at variance with the supposition that the- 



