ARCHETYPE OF SKULL 



The first gill-slit, or, as Rathke here prefers to call it, 

 pharyngeal slit, closes completely in snakes and in Urodeles. 

 It forms the Eustachian tube in all other Tetrapoda. As 

 regards the vertebra;, Rathke describes them as being 

 formed in the sheath of the chorda from paired rudiments, 

 each of which sends two branches upwards, and two 

 branches downwards. The two inner pairs of processes 

 coalesce round the chorda, and later form the centrum ; 

 the upper outer pair meet above the spinal column ; the 

 lower outer pair form ribs. The odontoid process of 

 the axis vertebra is the centrum of the ~atlas (p. 120). 

 The formation of vertebral rudiments begins close behind 



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the ear-labyrinth, but in front of this the chorda-sheath 

 gives origin to a flat membranous plate which after- 

 wards becomes cartilaginous. This plate reaches forward 

 below the third cerebral vesicle as far as the infundibulum. 

 The notochord ends in this plate, which is the basis cranii, 

 just at the level of the ear-labyrinth. In no Vertebrate does 

 the notochord extend farther forward (p. 122). The basis 

 cranii gives off three trabeculae. The middle one is small 

 and sticks up behind the infundibulum ; it is absent in fish 

 and Amphibia, and soon disappears during the development 

 of the higher forms. The lateral trabeculae are long bars 

 which curve round the infundibulum and reach nearly to the 

 front end of the head. Together they are lyre-shaped. The 

 cranial basis and the trabeculae are formed, like the vertebrae, 

 in the sheath of the notochord, and the only differences 

 between the two in the early stage of their development are 

 that the formative mass for the cranial basis is much greater 

 in amount than that for the vertebrae, and that the cranial 

 basis by means of its processes, the trabeculae, reaches well 

 in front of the terminal portion of the notochord (p. 36). 

 The capsule for the ear-labyrinth develops quite independently 

 of the cranial basis and the notochord. It resembles on its 

 first appearance, in form, position, composition, and con- 

 nections, the ear-capsule of Cyclostomes, and so do the ear- 

 capsules of all embryonic Vertebrates (p. 39). It manifests 

 clearly the embryonic archetype, ..." there exists one 

 single and original plan of formation, as we may suppose, 

 upon which is built the labyrinth of Vertebrates in general" 



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