HOMOLOGY. 



19 



Fig. 8. 



upon tlie evidences afforded by mature Vertebrates at the 

 lower end of their scale for any retention of this character — 

 a passing one — in the higher forms. 



Fishes, especially the cartilaginous, yield 

 such illustrations. I may refer to Busch's 

 descriptions and figures of piscine brains 

 exemplifying such suggestive characters, in 

 his excellent monograph ' De Selachiorum et 

 Ganoideorum Encephalo ' *, from which the 

 illustrations of such character in the brain of 

 the Sturgeon (Acipenser sturio), and more 

 especially in that of the ChimcBra monstrosa, 

 are taken, in figs. 173 and 179 of my 

 ' Anatomy of Vertebrates.' The long cord- 

 like lamellae continued from the optic lobes 

 (fig. 8, 4) to the cerebral one [ib. e), equal in 

 longitudinal extent both mes- and prosen- 

 cephalon combined. The so-termed " third ^^^^^ 

 ventricle " appears as an elongated widely open channel, the 

 side walls of which {ib. 5, 5) are thickened and, expanding 

 into the cerebral hemisphere, seem to represent the " crura 

 cerebri." They indicate that these so-called cords or tracts, 

 in Vertebrates, may be homologous with the parial cords 

 or tracts girting the gullet and connecting the fore brain 

 (fig. 4, 6) with the hinder masses (ib. s) in Invertebrates ; to 

 which pair of intercommunicating tracts the oral end of the 

 gullet in Invertebrates and the conario-hypophysial tract in 

 Vertebrates hold like relations. 



In Birds the primitive oesophageal tube is partially obli- 

 terated by ingrowths of a vascular tissue, the folds of which 

 have a branched character, recalling the structure of a choroid 



* 4to, 1848. 



c3 



