DEVELOPMENT OF ELASMOBRANCH FISHES. 183 



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 con- 

 clusions arrived at are in part opposed to my own results. 

 Professor Wilder says : " The true hemispheres are the lateral 

 masses, more or less completely fused in the middle line, and 

 sometimes developing at the plane of union a buodle of longi- 

 tudinal 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 Mustelus at 

 least seem to he formed after them'' 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 conclusions 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 (1) the olfactory lobes (crura and bulbs) are direct 

 outgrowths from the cerebral rudiment, and develope quite 

 independently 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 sack. In some other Vertebrates this nerve seems 

 hardly to be developed, but it is easily intelligible, that if 

 in the ordinary course of growth the olfactory sack became 

 approximated to the olfactory lobe, the nerve which grew out 

 from the latter to the sack might become so short as to escape 

 detection. 



