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Mr. W. K. Parker on the Development of the [Feb. 13 y 



extension of the dorsal part of the mandibular arch suppresses the 

 pre-oral cartilages ; in the Selachians — sharks especially — this is done 

 by the pterygoid outgrowth of the same element. 



In the first case, the whole mouth is dominated by the suctorial car- 

 tilages; in the second, the immense development of the pterygoid 

 process of the manibular pier, antagonising the movable lower jaw r 

 causes the great suppression of the proper maxillo-palatine fold. 



Whilst I consider the trabeculee to be axial, or rather neuro-axial, I 

 am very familiar with the rudiments of a visceral arch growing from 

 their fore ends. 



I first saw this in metamorphosing larvae, and adult, bull-frogs ; 

 afterwards Professor Huxley found and described them in Rana tem- 

 porta (" Encyc. Brit., vol. ix, Art. Amphibia, p. 755). Since then, 

 I have found them in many kinds of Batrachia ; in some Urodeles ; in 

 the embryos of the dog-fish (Scyllium canicula) ; in passerine birds 

 and in the Mammal. 



Their largest development, however, is in the " Bolocephalous " 

 fishes — Chimcera and Callorhynchus. 



These three pairs of rudimentary pre-oral arches are like the 

 first post-oral of the Mammal, non-segmented; they are sometimes 

 direct outgrowths of, and at others are formed separately from, the 

 basal bar (trabecular). 



I cannot see that these cartilages are anything else than arrested 

 representatives of the large, fully developed post-oral arches; they 

 correspond, however, only with the upper or suspensorial segment. 



The great difference between the head and body in existing Verte- 

 brates is shown in every part composing these two regions. 



Therefore, any impatient premature attempt to make a perfect har- 

 mony between the parts that form the axial, the neural, and the heemal 

 regions of each, will end in disappointment. 



Embryology must show us how true is the deep, essential, primary 

 homology of these parts ; but morphology must come in and demon- 

 strate the great and inherited differences, slowly arising, no doubt, 

 that are to be seen between the two regions. 



There is a real generic likeness between the axis, the upper and 

 lower arches, and the overlying parts in the cranial and spinal regions ; 

 let these be computed at their true worth in any comparison of the two 

 categories one with another. 



Thus, the divided basi-neural regions of the body are comparable 

 with the continuous basi-neural regions of the head, and the visceral 

 arches may be likened to ribs. 



The "uncinate processes" of the ribs of certain reptiles (Hatteria 

 and the Crocodilia), and in all birds, except Palamedea, are com- 

 parable to the branchial "rays" of Selachians; and the overlying 

 " extra-branchials " are not void of a true similitude to the girdles of 



