AUG 2L 1903 r .r- -, 



XII. On the Morphologn of the Cerebral Commissures in the Vertehrata, with Special 

 Reference to an Aberrant Commissure found in the Forebrain of certain Reptiles. 

 By G. Elliot Smith, M.A., JI.D., Fellow of St. John's College, Cambridge ; 

 Professor of Anatomy, Egyptian Government School of Medicine, Cairo. {Com- 

 municated by Prof. G. B. Howes, B.Sc, LL.D.. F.B.S., Sec. Linn. Soc) 



(With 36 Illustmtions in the Text.) 



Read nth. June, 1902. 



In spite of the innumerable memoirs which have been published on the homologies of 

 the cerebral commissures in the Sauropsida and Ichthyopsida, the essential problem has 

 practically remained untouched, and the subject still awaits a full discussion. I do not 

 propose at this time to attempt the complete solution of this matter, but merely wish to 

 point out the natm-e of the question at issue, and to bring forward some facts which seem 

 to suggest in which direction we must seek for the full explanation. 



To make clear the exact scope of this communication, I may be permitted to explain 

 the circumstances under which it was written. 



About live years ago the late Mr. Martin Woodward gave me the head of an adult 

 Sphenodon (preserved in alcohol), from which I was able to obtain a brain in a sufficiently 

 good state of preservation to l)e cut iu a series of coronal sections. When these had been 

 stained with lithium carmine, I was greatly surprised to find a band of commissural 

 fibres crossing the epithelial roof of the third ventricle midway between the " dorsal " 

 (hippocampal) and "superior" (habenular) commissures, such as I had hitherto seen 

 oulv in the Lacertilia. [In the following account I shall call this commissiu'c b^s' the 

 name ^' aberrans," because all the other terms in current use are highly objectionable.] 



Shortly afterwards I was enabled to confirm this fact, and study the relations of the 

 coinmissura aberrans in a series of sagittal sections, wliich were cut from a specimen 

 kindly given to me by Dr. Hans Gadow of Cambridge. 



Although the only contributions * to our knowledge of the l)rain of Sphenodon w^hieh 

 had been published at that time contained no reference to the presence of the pecuhar 

 commissure, I withheld the publication of ray incomplete studies, in the hope of 

 obtaining material which might be examined after treatment by the staining methods of 

 Weigert and Golgi. 



In 1899 Professor Arthur Deudy, for the first time, placed on record the existence ol' 

 this peculiar commissural tract in Sphenodon f. Dendy's memoir was written in Xew 



* Bald-nTii Spencer, " On the Presence and Structure of the Pineal Eye in Lacertilia," Quart. Journ. Micr. Sci., 

 vol. xxvii. p. 165; G. Osawa, "Beitriige zur Anatomie der Hattevia puiictata." Archiv mikr. Anat., Bd. li. p. 587; 

 R. Wiedersheim, Grundriss d. Vergl. Anatomie. 1893. p. 2til. 



t A. Deudj-, '• The Parietal Eye and Adjacent Organs in HjAenodon," Quart. Journ. Micr. Sci., vol. xlii. pp. 121 

 and 143. 



SECOND SERIES. — ZOOLOGY, VOL. VIII. 67 





