THE DEVELOPMENT OF THE FROG 269 



composed of a pair of curved longitudinal bars, the trabeculse 

 cranii, lying below the fore-brain, which are soon continued 

 backwards into a pair of narrow parachordal plates lying on 

 either side of the anterior end of the notochord. The middle 

 and hinder parts of the trabecule diverge from one another, 

 leaving between them a space through which the pituitary body 

 passes. The parachordals unite with one another above and 

 below the notochord, and their hinder margins grow up round 

 the hind brain and eventually meet and fuse above to complete 

 the occipital region of the cranium. The trabeculse unite in 

 front of the pituitary fossa to form the floor of the fore-brain, 



A, The cranial and mandibular cartilages of a tadpole seen Trom above 

 (semi-diagrammatic). B. The skull and branchial arches of ah older 

 tadpole seen from the side. tzn, auditory capsule ; 3r, branchial 

 arches ; c/iy, ceratohyal cartilage ; c^r, horns of the trabeculse ; /c, 

 labial cartilage ; mk, Meckel's cartilage ; na, olfactory chamber ; 

 nch, notochord ; /c, parachordal ; pit, pituitary fossa ; p^t, palato- 

 pterygoid bar. ^, quadrate ; so, subocular arch ; st, stapes ; t?; tra- 

 ' becula; ; 3, optic foramen ; 5, foramen for fifth nerve. (B, after Parker.) 



and their margins, and those of the anterior part of the para- 

 chordals grow up to form the side walls of the cranium, but 

 do not meet above, so that the roof of this region is imperfect. 

 The pituitary fossa is eventually closed and the anterior ends 

 of the trabeculae are fused with the cartilaginous capsules of 

 the olfactory organs, while the capsules of the auditory organs 

 are fused with the upgrowths of the parachordals in the 

 posterior region of the cranium. The various bones of the 



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