406 DEVELOPMENT OF ELASMOBRANCH FISHES. 



importance, and may not improbably be regarded as a real 

 ancestral feature. Some observations have recently been pub- 

 lished by Professor B. G. Wilder 1 upon this point, and upon the 

 homologies and development of the olfactory lobes. Fairly good 

 figures are given to illustrate the development of the cerebral 

 hemispheres, but the conclusions arrived at are in part opposed 

 to my own results. Professor Wilder says : " The true hemi- 

 spheres are the lateral masses, more or less completely fused in 

 the middle line, and sometimes developing at the plane of union 

 a bundle of longitudinal commissural fibres. The hemispheres 

 retain their typical condition as anterior protrusions of the 

 anterior vesicle ; but they lie mesiad of the olfactory lobes, and 

 in Mnstelns at least scan to be formed after tJiem" The italics 

 are my own. From what has been said above, it is clear that 

 the statement italicised, for Scyllium at least, completely reverses 

 the order of development. Still more divergent from my con- 

 clusions are Professor Wilder's statements on the olfactory lobes. 

 He says : " The true olfactory lobe, or rhinencephalon, seems, 

 therefore, to embrace only the hollow base of the crus, more 

 or less thickened, and more or less distinguishable from the main 

 mass as a hollow process. The olfactory bulb, with the more or 

 less elongated crus of many Plagiostomes, seems to be developed 

 independently, or in connection with the olfactory sack, as are 

 the general nerves ;" and again, " But the young and adult brains 

 since examined shew that the ventricle (i.e. the ventricle of the 

 olfactory lobe) ends as a rounded cul-de-sac before reaching the 

 ' lobe.'" 



The majority of the statements contained in the above 

 quotations are not borne out by my observations. Even the 

 few preparations of which I have given figures, appear to me to 

 prove that (i) the olfactory lobes (crura and bulbs) are direct 

 outgrowths from the cerebral rudiment, and develope quite in- 

 dependently of the olfactory sack ; (2) that the ventricle of the 

 cerebral rudiment does not stop short at the base of the crus ; 

 (3) that from the bulb a nerve grows out which has a centrifugal 

 growth like other nerves of the body, and places the central 

 olfactory lobe in communication with the peripheral olfactory 



1 "Anterior brain-mass with Sharks and Skates," American Journal of Science 

 and Arts, Vol. xn. 1876. 



