OI' THE CONAEIO-HTPOPnrsiAL TRACT. 



141 



derived and caudallj continued the liomologue of tlie Vertebrate 

 myelon {ih. i), together with the part of the gullet they encompass, 

 are consequently the homologues of the parts of the brain (fig, 2, 

 4, 3) posterior to the cerebrum (ih. e) and of the ventricle (fig. 4, 5) 

 intervening between the upper and lower ends — pineal {ih. 7) and 

 pituitary {ih. s) — of the conario-hypophysial tract. Thus, as it 

 appears to me, is the Unity of Organization or Composition vindi- 

 cated, though in a transitory manner, between the Yertebrate and 

 Invertebrate brain-possessing animals. The foregoing develop- 

 mental phenomena have mainly guided me to a homological appli- 

 cation which, so far as my readings have extended, appears not to 

 have suggested itself. 



An obvious difference from the mature Vertebrate is the rela- 

 tive extent of the interspace dividing the fore brain from the 

 mid brain, depending chiefly on the functional relations of the 

 interposed alimentary canal in Invertebrates (fig. 3, 7-10). The 

 proposed homology appears to me to throw some welcome light on 

 the similar though transitory proportions of the same interspace 

 in the Vertebrate, even the Mammalian, embryo, as exemplified in 

 fig. 4. i\nd we now look with interest upon the evidences afforded 

 by mature Vertebrates at the lower end of their scale for any reten- 

 tion 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 Selachi- 

 orum et Grauoideorum Encephalo'*, from 

 which the illustrations of such character 

 in the brain of the Sturgeon {Acipenser 

 sturio), and more especially in that of the 

 Cliimcera monstrosa, are taken, in figs. 173 

 and 179 of my 'Anatomy of Vertebrates.' 

 The long cord-like lamellae continued from 

 the optic lobes (fig. 6, 4) to the cerebral one 

 {ih. c), equal in longitudinal extent both 

 mes- and prosencephalon combined. The 

 so-termed " third ventricle " appears as an 

 elongated widely open channel, the sidewalls 

 of which {lb. 5, 5) are thickened and, expand- 



* 4to, 1848. 



Fiff. 6. 



Brain of Chivufra. 



