546 COMPARATIVE ANATOMY. 



backwards, that when the last has appeared the anterior ones have 

 already undergone certain changes. They are all gradually 

 atrophied, and completely disappear, except the first, part of which 

 is converted into the middle and outer ear (cf. supra, § 402). 



The degeneration of the embryonic branchial clefts is an impor- 

 tant point of difference between the Amniota and the Anamnia, but, 

 in addition to this, there is a new peculiarity which is due to a 

 differentiation of the primitive buccal cavity. This leads to the 

 formation of the secondary nasal cavity, and of the secondary 

 buccal cavity. The remnant of the primitive buccal cavity, which 

 lies behind, and is not affected by this process, forms the Pharynx. 

 The cartilaginous portion of the ethmoid, which separates the two 

 nasal cavities, and is broad in the Amphibia, is developed in the 

 Amniota into a thin vertical lamella (Fig. 307, e) — the internasal 

 septum. It remains partly cartilaginous, and is partly converted into, 



and develops bony structures, which were 

 treated of under the cephalic skeleton. 



A second change is brought about by hori- 

 zontal ridges or processes, which are given off 

 from the maxillary process of the first arch, 

 and which gradually form a plate (Fig. 307, p), 

 the palate, which divides the primitive buccal 

 cavity into two compartments. This plate 



Fig. 307. Diagram o£ ^^^'^^ *^® ^°°^ °^ *^^ upper. Or nasal cavity (h), 



the differentiation of the and the roof of the lower one (m). When the 



primitive buccal cavity internasal septum reaches this palatine plate 



intx) nasal cavities (w n) ^^ separates the nasal cavity into two por- 



and a secondary buccal ,. ^ . , , „ i-iji ii 



cavity (m). p Palatine tions, mto each 01 which the nasal canal 

 plate, c Internasal wall, now Opens, while its external orifice is coin- 

 cident with that of the bifid nasal cavity. The 

 posterior orifices of the nasal cavity, the choanee, which are 

 separated by the palatine plate from the buccal cavity, and by 

 the vertical internasal wall from one another, open into the 

 pharynx. 



Very various stages in the arrangement of these palatine plates 

 may be observed. In the Ophidii, Saurii, and Aves, the process of 

 separation is less complete, the posterior nares form a longitudinal 

 cleft, owing to the palatine processes uniting anteriorly, but being 

 separated from one another posteriorly. They are sometimes 

 separate in Birds, in which case they are exceedingly small. In the 

 Crocodilini they are placed farther back than in any other forms, 

 while in the Mammalia they do not open into the secondary buccal 

 cavity, but into the pharynx. This latter region is thereby — as also 

 by the opening into it of the Eustachian tube, which is developed 

 from the first visceral cleft — shown to be a portion of the primitive 

 respiratory antechamber. 



In Eeptiles and Birds the palate is supported by pieces of the 

 Skeleton (vide supra) ; in Mammals the hinder portion is formed 

 of soft parts, which form the " velum palatinum.'^ 



