( 487 ) 
XVIII. — The Morphology of the Prosencephalon of Spinax as a type of Elasmo- 
branch Fore-brain. By J. Stuart Thomson, M.Sc., Ph.D., Lecturer and 
Senior Demonstrator in Zoology in the Victoria University of Manchester. 
(With Two Plates and Three Text-figures.) 
(MS. received January 26, 1918. Read February 4, 1918. Issued separately April 25, 1919.) 
CONT 
PAGB 
Introduction and Gross Morphology . . . 487 
Part I. — Gray Masses. 
Tuberculum olfactorium 489 
Formatio pallialis 489 
Primordium hippocampi ..... 489 
Corpus striatum ....... 489 
Corpus paraterminale 489 
Part II. — Fibre Tracts. 
Tractus olfactorius ...... 493 
Tractus olfacto-corticalis lateralis . . . 493 
Tractus olfacto-corticalis medio-dorsalis . . 493 
NTS. 
PAGE 
Tractus cortico-medialis 494 
Tractus olfacto-hypothal amicus .... 495 
Tractus taeniae 495 
Tractus pallii 495 
Commissura hippocampi 495 
Commissura pallii posterior ..... 496 
Tractus medianus 496 
Tractus strio-thalamicus (anterior commissure and 
medial fore-brain bundle) 497 
Tractus thalamo-corticalis 497 
Fibre complexes . . . . . . . 498 
Nerve fibre tracts (summary) .... 499 
Explanation of plates ...... 500 
Introduction and Gross Morphology. 
This investigation on the morphology of the Fore-brain or' Prosencephalon of 
Elasmobranchii was suggested to me by Professor G. Elliot Smith, F.R.S., and I 
am indebted to him for introducing me to an interesting if somewhat difficult and 
perplexing subject. A glance at the literature on the morphology of the prosen- 
cephalon will readily convince one of two things — firstly, of the endless field for work 
in endeavouring to ascertain the true homologies of the neurone areas and fibre tracts 
in the brain throughout the Phylum Vertebrata ; and secondly, of the somewhat 
unnecessary obstacles which have been introduced into the problem by the more 
recent additions to a terminology, already overburdened and encumbered, of new 
terms for equivalent parts. It would appear to be very advantageous if mutual 
agreement could be arrived at among workers on this subject regarding the use of 
descriptive terms, and, if such is at present impracticable, that each new worker 
should refrain as far as possible from applying a new term to a structure already 
previously described. In one case, namely, that of the tuberculum olfactorium, five 
different terms have apparently been applied to the same area. I obtained a large 
amount of material for my study of the brain of Elasmobranchs during periods of 
work at the marine biological stations of Naples, Plymouth, and Cullercoats, and to 
the authorities at these institutions I have pleasure in acknowledging my cordial 
TRANS. ROY. SOC. EDIN., VOL. LII, PART II (NO. 18). 74 
