No. 2.] VERTEBRATE CEPHALOGENESIS. 223 



the inclusion of vertebral remains within the skull, nor is this 

 view influenced by the fact that such inclusions are known to 

 exist, e.g. in the cartilaginous fishes, etc., and for this reason ; the 

 skull is formed about the brain and includes various organs, not 

 always the same, within its substance. As illustrations of the 

 variations met with, we have in some cases the aorta included 

 in the cartilaginous basis cranii, though usually it is left out, 

 the notochord usually for the whole of its cranial length, though 

 it may be left out in part, as for example its anterior end may lie 

 below the cartilage or above, i.e. inside the cranial cavity either 

 projecting beyond the pituitary prominence or running out on 

 the inner face of the skull behind it. There are other facts of 

 this class which have not received enough attention. 



A. TJie miterior end of the neural axis of Amphioxus is a 

 brain, and corresponds zvitJi a certain definite portion of the 

 brai7is of other vertebrates. Its anterior wall is the homologne 

 of the lamina terminalis of other vertebrate braijis, a7id the ante- 

 rior portion of its unpaired ventricle is the thalamoccele. There 

 is a posterior portion of the ventricle intimately associated with 

 a ganglionic tract, which corresponds to the mesoccele, tuhile the 

 myelocoele remains, 7nore or less widened, varying much in 

 different individuals, but always in an undijferentiated condition. 



Although the nervous system of Amphioxus has been often- 

 times studied, many features of importance remain to be de- 

 scribed, and the well-established relations deserve reconsideration 

 in connection with the new ones. With the improvements in 

 methods of preparation, results unobtainable otherwise are 

 reached, which show that we may still hope for more light on 

 important questions from even well-worked fields. The prepa- 

 rations I have studied were made by the celloidin method of 

 imbedding and nitric acid maceration process, and I can recom- 

 mend both as giving excellent results. 



I may say, to begin with, that I do not consider Amphioxus 

 to be a degraded form, in the usual sense of that word as 

 employed in vertebrate phylogeny. Its larval modifications 

 cannot be called degradations of structure, nor are its adult 

 peculiarities to be looked upon in the light of degraded or 

 bizarre modifications of normal vertebrate conditions ; on the 

 contrary, they are to be regarded as not only ftormal, but also 



