10 PEOF. OWEN" ON CEEEBBAL HOMOLOGIES 



of the entire frame in the two groups. If the ganglionic cord be 

 the homologue of the myelon, the surface of the body next to 

 which those nerve-centres respectively extend must be the same. 

 If such surface be turned downward in the ordinary station and 

 progression of an Insect, the columns on which the sensory gan- 

 glions are formed will be " inferior ;" while in Vertebrates, accord- 

 ing to the position in which the body may be carried, the gangli- 

 onic or sensory columns will be " superior " in the beast and " pos- 

 terior " in the man. Terms, therefore, defining aspect and position 

 independent of the accident of limb-direction, should be accept- 

 able : " neural " and " haemal " are as applicable to parts as to 

 wholes. 



A heart, whether compact or elongate, has a surface looking 

 toward the " neural aspect," and a surface with an opposite aspect. 

 One may predicate of the hsemal side of a "heart " or " dorsal 

 vessel " whether it be at the fore side of the body (in a man), or at 

 the under side (in a beast), or along the upper side (in an insect). 

 So likewise with regard to the nervous axis : Newport's sensory 

 ganglions in that of the Insect are developed in and from the 

 cords on the "neural" side of such axis, as they are in the 

 " neural " columns of the Vertebrate myelon, as distinguished 

 from the " hsemal " columns. 



Developmental researches may gain by such appreciation. The 

 admirable Investigator whose recent loss morphologists deplore, 

 thus writes : — " The embryo of Peripatus shows what was once 

 part of a continuous slit running nearly its whole length ;"..." it 

 at first leads into the alimentary canal, like the neurenteric canal 

 of the vertebrate embryo ; but this communication is closed prior 

 to the appearance of the first rudiments of the ventral nerve- 

 cords"*. 



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

 lary canal, occupies the same position or aspect of the body in the 

 vertebrate embryo as does the so-termed ventral position in Peri- 

 patus — 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 MoUusks are the parts known 



in Articulates as the " supra-" and " suboesophageal ganglions " 



with their commissural or annectant cords or " crura," that 



* Balfour, ' Comparatiye Embryology,' 8vo, 1881, vol. ii. p. 312. 



