CEREBRAL HOMOLOGIES. 



45 



The primitive streak, or slit, prior to its closure as the 

 medullary canal, occupies the same position or aspect of the 

 body in the vertebrate embryo as does the so-termed ventral 

 position in Peripatus — that, namely, which in Vertebrates is 

 called " dorsal " as arbitrarily as in Invertebrates it is called 

 " ventral.'' It is the homologous aspect or position of the 

 body in both. 



But, to resume, my contention here is, that the homologues 

 of the primary divisions of the brain in Mollusks are the 

 parts known in Articulates as the " supra-" and " sub-oeso- 

 phageal ganglions " with their commissural or annectant 

 cords or "crura," that the topical relations of these parts to 

 the gullet are the same in both great divisions of Inverte- 

 brates, and that the homologies of the aforesaid parts with 

 the primary divisions of the Vertebrate brain are affected 

 solely by the altered relation thereto of the gullet and 

 mouth. 



The homologies of the Dibranchiate brain, notwithstanding 

 the oesophageal and oral differences and a non-appreciation 

 of their essential nature and cause, were recognized and 

 affirmed by the Father of the anatomy of the Mollusca. They 

 are clearly expressed in the first of his immortal ' Memoires'* 

 on that subject ; and are briefly summarized in the ' Le9ons 

 d' Anatomic comparee.' After describing the " sub-" and 

 " supraoesophageal" centres, Cuvier affirms: — "On pourrait 

 comparer le premier au cervelet, I'autre au cerveau des Ver- 

 tebres." If for " cerebellum " one writes " epencephalon," 

 this defined correspondence of the brain of the highest 

 Mollusks with that of the lowest Vertebrates would square 

 with my own convictions. 



* Memoires pour servir a I'llistoire et rAnatomie des MoUusques,' 4to, 

 1816, Mem. l'^'^, " Sur le Poulpe (Octopus vulgaris)" 



