OF THE CONAEIO-HTPOPHTSIAL THAC'f. 



139 



vocal traces of a branchial organizatioB, as sliown in fig. 4, lo. 

 But, altbougli tliis organization subsides, the bgemal mouth, or 

 " tritostome," is in them retained. 



Having noted, briefly, the indications of an earlier or neural 

 month-wav, or oesophagus, in the embryo o£ Yertebrates next 

 above the brainless Ampliioxiis, I may premise that, with the 

 appearance, in Invertebrates, of a brain includiug both supra- 

 (fig. 3, 6) and sub- (ih. s) oesophageal masses or ganglions — better 

 termed, respectively, " hsem oesophageal" and " neuroesophageal " 

 — the canal dividing them is developed as a " gullet" (ib. lo), and 

 its outward opening is established as a " mouth " {ih. i) or " deuto- 

 stome." 



In cerebral Vertebrates, also, there appears a beginning, or 

 attempt so to speak, of a caual or tubular extension directed 

 brainward. This develop- 

 mental phenomenon is con- 

 temporaneous with the en- 

 largements of the fore end 

 of the myelencephalon, as 

 seen in the embryos of 

 Cyclostomous fishes *. In 

 fig. 5 these enlargements 

 are represented by the 

 fi-gures 4, 5, 6, the latter now 

 pushing bej^ond the noto- Enlarged scale of a longitudinal rertical mid- 

 chord,2. Toward the middle section of an embryo of Fetromyzon Plancri 

 one of these extends the at the eighteenth day. 

 tubular production (s') from the digestive sac (ii). In the line 

 opposite to the production (s') is an infolding of the ectoblast 

 (7), which Scott indicates as that of the nasal cavity and hypo- 

 physis (" gemeinsame Einbuchtuug fur ISTasengrube und Hypo- 

 physis," loc. cit. p. 171, Taf. ix. fig. 31, N. h. e). Beneath this 

 has commenced the wider infolding of the ectoblast (fig. 5, 9'), 

 which, extending backward, and subsequently expanding and 

 developing the branchial sacs, ultimately effects a communica- 

 tion with the alimentary cavity (n), and establishes the perma- 



* See Owsjannikow, " Die Entwicklungsgeschichte derPefromi/zonfluvia^ilis," 

 Bulletin de I'Acad. Imp., St. Petersbourg, torn. xiv. 1870, p. 325; Calberla, 

 Morpholog.Jahrbuch, Bd. iii. p. 226 ;. Scott (W. B.), Morpholog. Jahrbuch, Bd. 

 vii., erstes Heft, p. 131, " Beitrage zur Entwicklungsgeschichte der Petromy- 

 zonten," from which treatise the subject of fig. 5 is taken. 



10* 



