298 



COMPARATIVE ANATOMY 



VIZ., the blastopore (Urmund), the neuropore (the anneUdan mouth), 

 and finally the definitive mouth." According to this view, the neural 

 tube was formerly a part of the digestive system, and its anterior embryonic 

 external opening, the neuropore, once functioned as a mouth. For a part 

 of the digestive system to become nervous in function is indeed a sur- 



BLASTOPORE 



ENDODERM 



A.GASTRULA 



GILL POUCHES 



B.AMPHIOXUS EMBRYO 



NOTOCHORD 



BLASTOPORE 



ENTERON 



MOUTI 



NEUROPORE 



^PINALCORD ^^HOPj, 



GILL POUCHES 



C.UROCHORDATE LARVA 



ENOODERM STRAND 



NEUROPORE 



OTIC CAPSULE 



NEUR- 



NOTOCHORD 



ENTE 



RIC CANAL , 

 (BLASTOPORE) 



POST-ANAL GUT 



ANUS 



GILL POUCHES 

 DEFINITIVE MOUTH 

 HYPOPHYSIS 



D. VERTEBRATE 



Fig. 249. — Diagrams illustrating the hypothetical phylogenesis of the vertebrate 

 mouth. The primitive animal mouth, the blastopore, is converted in vertebrates either 

 into an anus or a neurenteric canal. The definitive mouth of vertebrates therefore is a 

 secondary mouth. But the relations of the neuropore are such that at one time in the 

 ancestry of chordates this may have served as a mouth and the neural tube as a foregut. 

 It is also possible that the mouth of urochordates is not homologous with the definitive 

 mouth of vertebrates. The evidence of a paleostoma or hypophysial opening suggests 

 that this may once have been a functional mouth. Thus the definitive mouth may have 

 been the last in a series of four mouths. 



prising assumption, yet scarcely more so than many other transformations 

 assumed by evolutionists. The digestive apparatus postulated by 

 Delsman on the basis of the relations presented in the Amphioxus embryo 

 has seemed to zoologists too impractical for daily use by any adult animal. 

 The notion that the neuropore of chordate embryos represents a former 



