788 Comparative Morphology of Chordates 



order of existing vertebrates is most definite. A palaeencephalon ex- 

 tends throughout the series, with only relatively minor modifications. 

 The elaboration of a neencephalon or "superbrain" is at its minimum 

 in fishes, particularly in the Teleostei. Elasmobranch fishes, amphib- 

 ians, and reptiles show, in the order mentioned, progressive increase 

 in the importance of the neencephalic structures, especially the pallial 

 cortex. The brain in birds is essentially reptilian in structure, the chief 

 difference being greater complexity of the palaeencephalic structures 

 concerned with the very highly developed instinctive behavior of the 

 bird. In the mammalian brain the neencephalic parts are at the maxi- 

 mum for the vertebrate series. 



Not only these several major features which have just been briefly 

 reviewed but also innumerable details of vertebrate structure (espe- 

 cially the skull and other skeletal parts) show differences in degree and 

 nature of their elaboration which, if made the basis of an arrangement 

 of existing vertebrates, cause them to fall into line in the order, fishes — 

 amphibians — reptiles, and at "reptiles" the series bifurcates into two 

 lines, one mammalian, the other avian. 



Comparative Embryology corroborates the serial order of the 

 vertebrates as determined by Comparative Anatomy, and adds 

 some highly relevant facts. The earlier developmental stages of all 

 vertebrates exhibit an essential similarity in structure and in the 

 manner in which the embryonic parts develop. Certain of the early 

 embryonic organs — e.g., the notochord, pharyngeal pouches, and 

 aortic arches — attain complete and permanently functional develop- 

 ment only in fishlike vertebrates. In later stages of embryos of other 

 vertebrates, these structures may be obliterated or may persist as 

 abortive "vestiges," or may undergo a more or less profound meta- 

 morphosis, as when the pharyngeal pouches of the most anterior pair 

 become converted into cavities of the middle ear. Also, in later em- 

 bryonic stages appear structures, not recognizably present in earlier 

 stages, which become distinguishing anatomic features of the adults. 

 It may fairly be said, therefore, that all vertebrates begin to develop as 

 if they were destined to become fishes. Upon this common early- 

 embryonic basis is superimposed the development of the distinctive 

 features of the adult. In the course of this later phase of development, 

 the basic structures of the early embryo may attain full elaboration, or 

 they may suffer reduction, suppression, or metamorphosis, and to them 

 may be added various "new" structures — e.g., lungs, metanephros, 

 feathers, hair. If modern vertebrates be arranged in a series determined 

 by the degree to which the basic embryonic structure persists in the 

 adults and by the number and nature of the structures secondarily 



