134 Thomas Henry Huxley 



that the adult skull is a modified vertebral column than 

 it would be to affirm that the vertebral colunui is a 

 modified skull." Taking the embryological facts, he 

 shewed that the skull arose out of elements quite differ- 

 ent from those of the vertebral column. The notochord 

 alone is common to both. The skull is built up of 

 longitudinal cartilaginous pieces, now known as the 

 '* parachordals" and " trabeculae," of sense capsules 

 enclosing the nose and ear, and of various roofing bones. 

 In the historical development of the skull three grades 

 become apparent ; a primitive stage, as seen in Amphi- 

 oxus, where there is nothing but a fibrous investment 

 of the nervous structures ; a cartilaginous grade, as 

 seen in the skate or shark, where the skull is formed 

 of cartilage, very imperfectly hardened by earthy 

 deposits ; a bony stage, seen in most of the higher 

 animals. He shew^ed that in actual development of 

 the higher animals these historical grades are repeated, 

 the skull being at first a mere membranous or fibrous 

 investment of the developing nervous masses, then be- 

 coming cartilaginous, and, lastly, bony. He made some 

 important prophetic remarks as to the probable im- 

 portance that future embryological work would give to 

 the distinction between cartilage and membrane bones 

 — a prophecy that has been more than fully realised 

 by the investigations of Hertwig and of others. Our 

 present knowledge of the skull differs from Huxley's 

 conception practically only in a fuller knowledge of 

 details. We know now that throughout the series 

 there is a primitive set of structures common to all ani- 

 mals higher in the scale than Amphioxus, and forming 

 the base and lateral walls of the skull. This is termed 

 the Chondrocranium, because it is laid down in carti- 

 lage ; it is composed of the separate elements which 



