320 EMBEYOLOGY OF THE LOWER VEETEBEATES ch. 



upper end with the chondrocranium and its dorsal end becomes 

 segmented off, as the palato-quadrate cartilage, from the larger 

 ventral portion — Meckel's cartilage — which forms the primitive 

 skeleton of the lower jaw. 



In the animals mentioned the lower jaw remains throughout life 

 connected with the cranium through the dorsal portion of the original 

 arch. This must be looked on as in all probability the primitive 

 mode of attachment of lower jaw to skull and such skulls may there- 

 fore be termed protostylic. 1 



Both in Lung-fishes and Urodele amphibians the palato-pterygoid 

 process is much reduced. In Urodeles it makes its appearance only 

 at a late stage of development and is of comparatively small size. 

 In Lepidosiren and Protofterus it has become eliminated almost 

 entirely from development, being represented for a short time by a 

 slight condensation of tissue which never becomes chondrified. This 

 is probably to be interpreted as a modification of development 

 induced by the precocious development of the bony skeleton of the 

 upper jaw which in the forms mentioned replaces functionally the 

 originally cartilaginous skeleton. 



The Elasmobranch . fishes do not exhibit this reduction of the 

 palato-pterygoid bar for this becomes the functional upper jaw. On 

 the other hand an important modification of development has taken 

 place in correlation with the fact that in these fishes the original 

 dorsal end of the mandibular arch has lost its primitive function of 

 suspending the jaw, this function having been taken over by the 

 enlarged dorsal end of the hyoid arch (Hyostylic type of skull). In 

 correlation with this the portion of the mandibular arch lying above 

 the pterygoid outgrowth is, all through development, greatly reduced. 

 It is apparently represented by the prespiracular cartilage, which 

 develops comparatively late. 



The mandibular arch makes its appearance in Acanthias 

 (Sewertzoff, 1899) as a C-shaped rod of cartilage lying in the rim of the 

 buccal opening on each side (Figs. 156 and 159). The lower half of 

 this segments off as Meckel's cartilage, while the upper half, which 

 develops from behind forwards, clearly represents the pterygo- 

 quadrate bar. This becomes continuous with and later articulated 

 towards its anterior end with the trabecula — a doubtless secondary 

 connexion with the cranium seeing (1) that it arises from the 

 anterior and later developed portion of the palato-pterygoid outgrowth 

 and (2) that in primitive sharks such as Notidanus, in Lung-fishes, 

 and in Urodele amphibians, the attachment of mandibular arch to 

 skull is further back in the auditory region — in fact in the region of 

 the original dorsal end of the mandibular arch. 



In the lower Vertebrates apart from those mentioned the 

 development of the cartilaginous mandibular arch takes place on 



1 Graham Kerr, 1908. Attention is drawn in this paper to the need of an additional 

 term to designate the more primitive type of so-called autostylic skull. A similar 

 suggestion had, however, already been made by Gregory (1904). 



