CONTRIBUTIONS TO THE FOREBRAIN MORPHOLOGY IN SELACHIANS.

A large number of investigators have during the past fifty years been busy studying the selachian brain. The earlier of these investigators usually studied the brain as a whole, without entering into closer examination of any particular part. The description of all the different external forms under which the selachians brain appears was for the moment the chief thing, although at the same time an endeavor could be observed to homologize as far as possible the various parts with similarly situated areas in higher and lower types, especially in Petromyzon, ganoids and amphibians. A problem deeply interesting the investigators of about 1890 was the question as to whether the fore-brain of the selachians should be considered paired or unpaired. From the embryological investigations it appeared that the cerebrum must be considered as originally an unpaired evagination from the rostral end of the forebrain. In primitive genera such as Chimaera and Hexanchus the telencephalon was distinctly paired, while in more specialized genera as Torpedo external pairedness could not in general be proved.