CHAPTER XIV. THE SKULLS OF THE FROG AND THE BOG. 



1. The Chondro-Cranium. We have already given 

 a description of the mammalian ^kull, and we have stated 

 where the origin of the several bones was in membrane, 

 and where in cartilage ; but a more complete comprehension 

 of the mammalian skull becomes possible with the handling 

 of a lower type. We propose now, first to give some short 

 account of the development and structure of the skull of 

 the frog, and then_to_show briefly how its development and 

 adult arrangement demonstrate tEiTnianimaliaii skuIFto be 

 a f ui ida mentally similar structure, complicated and dis- 

 guised by further development and readjustment. 



Fig. 80, A, shows a dorsal view of a young tadpole's cranium ; 

 the brain has been removed, and it is seen that it was 

 supported simply upon two cartilaginous rods, the trabeculse 

 cranii. Behind these trabeculse comes the notochord (the 

 fore-runner of the vertebral column), and around its 

 anterior extremity are paired tracts of cartilage, the 

 parachordals. These structures, underlying the brain, are 

 all that appears at first of the brain-case* In front, and 

 separate from the cranium, are the olfactory sacs; the 

 eyes lie laterally to the trabeculse, and laterally to the 

 parachordals are two tracts of cartilage, the Otic or auditory 

 capsules, enclosing the internal ear. 



Fig. 80, B, shows a more advanced phase of the same 

 structures. The trabeculse have met in front and sent 

 forward a median and lateral parts to support the olfactory 

 sacs, forming the rudiments of a pair of olfactory capsules. 

 They have also flattened out very considerably, and have 

 sent up walls on either side of the brain to meet above it 

 and form an incomplete roof over it. The parachordals 

 have similarly grown up round the hind-brain and formed 

 a complete ring. Further, the otic capsules are fusing 

 with the brain-case. With certain differences of form these 

 elements the trabeculse, the parachordals, and the otic 



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