429 



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 differen- 

 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- 

 fication takes place continuously on each side of the notochord, and 

 beyond it, the two trabeculae cranii, unlike anything in the spinal 

 column, extend along the base of the cranium. No distinct carti- 

 laginous centra, and consequently no intercentra, are ever developed. 

 The occipital arch is developed in a manner remotely similar to that 

 in which the neurapophysial processes are formed ; but the walls of 

 the auditory capsules, which lie in front of them, and which give 

 rise to some of the parts, most confidently regarded as neurapo- 



VOL. ix. 2 G 



