138 iPEOF, OWEN OS THE HOMOLOGf 



the walls of that cavity, save where the primitive yolk-canal, 

 fig. 4, 19, passes on to the shrunk vitellicle, now shut out as 

 an appendage, ultimately to be absorbed or cast off at birth. 

 Here, however, we have a primordial "month " and " gullet," or 

 parts holding functionally, though transitorily, those relations to 

 the digestive sac. The persistent indication of such course of the 

 embryonal food is called "umbilicus:" it points to one inlet of 

 food which has made way for another ; and that other will make 

 way for a third. As well devote pains and speculation to the 

 "function" of the navel as to analogous remnants of a later 

 communication with the alimentary canal, doomed likewise to 

 obliteration with concomitant solidification of parts. 



In low radiate forms of life. Medusa e. g., the vitelline entry, 

 or " protostome," is permanent; a "deutostome" may, in like 

 manner, appear as another step in the rising scale which is not 

 parted with. 



But to return to our Yertebrate grade. The alimentary tube, 

 parallel with the myelonal one, communicates or anastomoses there- 

 with at both ends ; a common canal thus results, but of which the 

 hsemal portion will be modified to give sustenance to the body, 

 the neural portion to the mind. In the course of differentiation 

 the caudal intercommunion is first abolished. The anterior end 

 of the alimentary tube (fig. 4, ii), extending forward, comes into 

 close contact and continuity with the canal which may be de- 

 scribed as commencing below at the " infundibulum," and as con- 

 tinuing upward by the third ventricle to the base or origin of 

 the pineal production of the thalamencephalon, which production, 

 perforating, as in the embryo Iguana, the soft lamellar basis of 

 the cranial roof-bones, is only arrested in its aim to form a mouth, 

 or " deutostome," at the vertex, by failing to overcome the resist- 

 ance of the superincumbent epithelial layer — such resistance being 

 encouraged by the processes now on foot to establish an external 

 communication, elsewhere, with the fore part of the alimentary 



canal. 



In AinpMoxus and its earlier or humbler relatives the Asci- 

 dians, a mouth, or oral passage, is formed, which opens behind into 

 a vascular expansion from which the alimentary canal is continued. 

 This branchial sac is on the under or haemal side of the fore part 

 of the neural axis, issuing, in the lower division of Vertebrates, 

 in the perfection of a water-breathing apparatus, and manifesting 

 in the embryos of the higher half of the subkiugdom unequi- 



