500 



MORPHOLOGY OF CEKEBRAL COMMlSSTHES IN YEBTEBKATA. 



The vexed questions of the morphology of the piscine types of brain hardly come 

 within the scope of this memoir. But the investigator who studies the brain of 

 Animocoetes or Pefroniyzon, in the light of the information which a comparison of the 

 Amniote and Urodele types of brain has yielded, cannot ha-\'e much difficulty in seeing 

 in the jjrevailing conditions in the Cyclostome an extremely simple and archaic form of 

 the common vertebrate type. This is well shown by the important memoirs of Studuicka 

 (" Beitrage zur Anatomic und Entwickelungsgescliichte des Vorderhirns der Cranioten," 

 Sitzungsberichte d. konigl.-bohmisch. Gesells. d. AVissensch., Math.-j)hys. Classe, 1895) 

 and Johnston ("The Brain of Fetromyzon," Joiu*n. Comp. Neurology, March 1902.). 



In conclusion, I must acknowledge with the deejoest gratitude the liberal help so 

 generously afforded me by Pi'ofessor Howes, who not only obtained for me the material 

 for the research, but also devoted a vast amount of time and energy in helping me to 

 accomplish this task. 



J/ist of Abh'eviations used in the Diagrams. 



U.S., aquaeductus Sylvii. 

 alv., alveus. 

 b.o., bulbus olfactorius. 

 c.a., commissura alx^rrans. 

 c.d., ,, dorsalis. 



c.h., „ liabeuulfe (superior). 



C.J)., „ posterior. 



C.V., „ veutralis. 



c.f., columna fornicis. 

 ce., cerebral hemisphere. 

 cbl., cerebellum. 

 dors., dorsal sac (part of epithelial roof of the 

 third ventricle). 

 ep., epiphysis. 

 fasc, fasciculus raarginalis. 

 f.d., fascia deutata. 

 /.//., fissura hippocampi. 

 f.M., forameu Mouroi. 

 hip., hippocampus. 

 hyp., hypophysis. 

 I.V-, lobus pyriformis. 

 l.t., lamiua tcrmiualis. 

 mam., corpus mammillare. 

 m.o., medulla oblougata. 

 ne., ucopallium. 

 M.O., nervus olfactorius. 

 n.IV., uervus trochlearis. 



(l.t., tlial.nmus opticus. 

 opt., tractus opticus. 



/;., jiallium. 

 pcn\, parapliysis. 

 para., corpus paraterminale. 

 p.o., peduuculus olfactorius. 

 p.s., parietal stalk. 

 p.c.f., posterior choroidal fold. 

 plx.I., plexus chorioideus lateralis. 

 plx.IL, lamiua chorioidea. 

 ph. III., plexus chorioideus ventric. tertii. 

 plx.IV., ,, „ „ quarti. 



q., corpora quadrigemina. 

 red., recessus iutercommissuralis. 

 rec.s., recessus superior. 

 ret., retina. 

 S.I., sulcus limitaus. 

 s.gr., stratum grauulosum fasciae dentatse. 

 St., corpus striatum. 

 ter., stria tcrmiualis. 

 tel., tela, or epithelial roof. 

 t.o., tuberculum olfactorium. 

 tuli., tuber cinereum. 

 V.I., veutriculus lateralis. 

 v.III., „ tertius. 



v.IV., „ quartus. 



