TKANSACTIOSS OF SECTION D. 787 



7. On the Cerebral Commissiires in the Marsupialia and Monotremata} 

 By Johnson Symington, M.I)., F.li.S.E. 



The author gave an account of the literature of the subject, discussing more 

 particularly the views of Owen and Flower. In 1837 Professor (now Sir Richard) 

 Owen published in the ' Philosophical Transactions ' a memoir, On the Structure 

 of the Brain in Marsupial Animals. In this communication it was asserted that 

 the brains of the Marsupialia differed from those of the placental mammals in 

 having no corpus callosum. In 1865 Professor W. H. Flower published a paper 

 in the ' Philosophical Transactions,' in which he controverted the conclusions of 

 Owen, and maintained that a corpus callosum was present in the Marsupialia and 

 Monotremata. 



The material used by the author in his study of the subject included the brains 

 of the Ornithorhyuchus, Echidna, and various Marsupialia. Serial sections were 

 mounted for microscopic examination and stained with carmine, or by the 

 Weigert-Pal method. Sections were also made for naked-eye examinations upon 

 brains hardened in M tiller's tluid. 



The conclusions arrived at agreed in general with those of Owen. 



In the Marsupialia and Monotremata the anterior commissure is as large as, and 

 generally much larger than, any other transverse commissure of the cerebrum, and 

 it unites the whole of the cortex of the two hemispheres, except the gyri dentati 

 and hippocampi_majores._ They have no true corpus callosum, and the superior 

 transverse commissure is simply a commissure for the gyri dentati and hippocampi 

 majores— the hippocarapal commissure of Owen. The peculiar position of this 

 commissure is due to the fact that the convolutions it unites extend much farther 

 forwards than they do in the placental mammals. 



The Monotremata and Marsupialia agree in the general arrangement of their 

 cerebral commissure, and both differ very distinctly from that found in the lower 

 placental mammals so far as examined. 



8. The Early Development of the Isopod Crustacea. 

 By Professor J. Platfair M'Mureich. 



9. Note on the Geographical Distrihution of Ascidians. 

 By Professor W. A. Heedman, D.Sc, F.B.S. 



When the large collection of Ascidians, both simple and compound, brouo-ht 

 home by the Challenger expedition is arranged according to the latitudes where 

 the specimens were collected, it is seen that the great majority belong to the 

 southern hemisphere, and most of them to the far south, such 'localitie's as the 

 Straits of Magellan, Kerguelen Island, and Bass Strait being especially rich. Then 

 high northern latitudes follow the extreme south, while tropical and semi-tropical 

 regions are, on the whole, poorly represented. Moreover, the species from these 

 high latitudes were mostly obtained in abundance, and the individuals are mosth' 

 of large size, so that not only in number of species, but also in actual mass, the 

 bulk of the collection is from the far south or north. Hence it seems obvious that 

 the Ascidians flourish best in comparatively cold waters. 



However, it might seem improbable that the Challenger collection, extensive 

 though it is both in species and in localities, would represent the distribution of 

 tlie group quite adequately and correctly, so I have lately gone over the recorded 

 geographical distribution of all the known species of simple and compound 

 Ascidians, with the result that although, as might be expected, the long-continued 

 and minute investigations of the shores of north-west Europe and the Mediterranean 

 have helped to raise the numbers for temperate seas, still a very considerable 



' The paper is printed in cxtenso in the Journal of Anatomy and Phtjsiolony for 

 October 1892. J JV 



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