Mr. T. n. lluxlcy on the Theory of the Vertebrate Skull 435 



on, each neural semi-arc or Neur apophysis, and passes upwards, into 

 the s\)ine or Metancurapophysis; downwards, to unite sooner or later 

 with the diaphysis, or diaphysis and paraphysis ; and outwards into 

 the diapophysis. 



It is doubtful whether the paraphysis appears as a distinct osseous 

 element in any Fertebrata above the class of fishes, in very few of 

 which even is it distinguishable in the adult state. Consequently 

 in the higher Vertebrata the paraphysial region is ossified, either from 

 the diaphysis or from the neurapophysis, or from both ; and a suture 

 exists for a longer or shorter time at the point of junction of the 

 neural and central ossifications. I will term this the Neiirocentral 

 suture. Its position is no certain or constant indication of the 

 nature of the parts above or below it, for it may vary in the same ver- 

 tebral column from the base of the neurapophysis to the junction 

 of the paraphysial with the diaphysial region of the centrum. 



The number of the centres of ossification in each distal portion of 

 the haemal semi-arc may vary greatly ; the uppermost is called a 

 Pleur apophysis, the lower, Hcemapophyses and Met-hcemapophyses. 



Besides these primary centres of ossification of a vertebra, there 

 are others of less constancy. Thus the ends of the metaneurapo- 

 physes, diapophyses, and zygapophyses in many Mammalia are ossi- 

 fied from distinct centres ; and in the caudal region of many of the 

 higher Vertebrata, outgrowths of the centra unite below to enclose 

 the caudal vessels, and ossify as distinct apophyses. 



If the development of the skull be now compared with that of the 

 spinal column, it is found that (1) the very earliest changes under- 

 gone by the blastoderm in each are almost identical. The primitive 

 groove extends to the extremity of the future cranial cavity ; its 

 lateral walls are continuous with the laminae dorsales, and these pass 

 into laminae ventrales, also continuous with those of the spinal region. 

 The laminae dorsales of the head become the cranial walls and en- 

 close the cerebrum — the continuation of the myelon ; the laminae 

 ventrales give rise to the boundaries of the future buccal and pha- 

 ryngeal cavities. 



2. But at this point the identity of the skull with the spinal 

 column ceases, and the very earliest steps in histological diflPeren- 

 tiation exhibit the fundamental differences between the two. For, 

 in the first place, in no instance save the Amphioxus, has the 

 notochord as yet been traced through the whole of the floor of the 

 cranial cavity. In no other embryo has it been yet seen to extend 

 beyond the middle vesicle of the cerebrum, or in other words, 

 beyond the level of the rudiment of the infundibulum and pituitary 

 body. 



In the second place, the division into somatomes, in all known 

 vertebrate embryos, stops short at the posterior boundary of the 

 skull, and no trace of such segmentation has yet been observed in 

 the head itself. 



3. Apparently as a consequence of these fundamental differences, 

 the further course of the development of the skull is in many 

 respects very different from that of a vertebral column. Chondri- 



28* 



