Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 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
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 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 of forms the floor of the UPP^ or nasal cavity («), the differentiation of the and the roof of the lower one (m). When the primitive buccal cavity internasal septum reaches this palatine plate into nasal cavities (»»), i{. separates the nasal cavity into two por- and a secondary buccal . r . , „ , . , J ,r cavity (m). p Palatine tions, into each oi 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 or
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