PROSENCEPHALON OE SPINAX AS A TYPE OF ELASMOBRANCH FORE-BRAIN. 489> 
prosencephalon are at first only slightly defined, but they gradually become more 
and more definite as one examines sections in a posterior direction. Fig. 8, which 
illustrates the condition in which the hemispheres are completely fused ventrally 
in the median line over an extent of about half their height, shows the various 
areas of neurones. The roof of the prosencephalon is occupied by a mass of 
neurones, which has been regarded as the forerunner of the hippocampus, and 
has therefore been termed the Primordium hippocampi ( d ). The Corpus para- 
terminale (e) is commencing to appear as a ventral extension in the medial direction 
of the Primordium hippocampi. The somewhat saucer- or cap-shaped area which 
runs latero-ventrally parallel with the margin of the section is the Tuberculum 
olfactorium (6). The mass of neurones which the latter partly encloses is the 
Corpus striatum (a). The area lateral to the dorsal half of the ventricle is the 
Formatio pallialis (c). 
If fig. 3 is compared with Elliot Smith’s* fig. 1, representing a coronal section 
through the left cerebral hemisphere of an adult Lepidosiren, the corresponding 
areas are readily recognised. In both the formatio pallialis is readily seen, and 
its medial continuation the hippocampus ; the tuberculum olfactorium running 
round the latero-ventral edges of the hemispheres is evident. The tuberculum 
olfactorium is, as described by Elliot Smith, a “ peculiar cortical formation cover- 
ing, like a closely fitting cap, the corpus striatum.” The paraterminal body is 
commencing to become defined as an extension in the ventral direction of the 
primordium hippocampi. 
These five main areas, as recognised in the Dipnoan cerebral hemispheres, may 
be followed in sections of the fore-brain of Spinax from the point depicted in fig. 3 
in an anterior and posterior direction. Fig. 3 also shows two grooves in the wall of 
each lateral ventricle ( pr',pr ,A ), opposite to which lie two more or less cell-free zones 
( x ' , x"). The groove has been termed the sulcus limitans, and the zone the zona 
limitans, and these may be further distinguished, according to their position, as 
lateral and medial. I have used the terms adopted by Elliot Smith in his 
description of the Dipnoan cerebral hemisphere throughout this paper. They imply 
a certain view regarding the structure of the fore-brain which I have not space to 
enter into here ; but this is on the whole unnecessary, as this conception has been 
repeatedly emphasised by Elliot Smith. t I would therefore reiterate that the 
main area of neurones which I recognise in the Elasmobranch fore-brain, as exempli- 
fied by that of Spinax, and as seen in my figures, are the following : the formatio 
pallialis, the primordium hippocampi, the corpus paraterminale, the tuberculum 
olfactorium, and the corpus striatum. 
* Grafton Elliot Smith, “The Cerebral Cortex of Lepidosiren,” etc., Anatomischer Anzeiger, Bd. xxxiii, Nos. 20 
and 21, Jena, 1908. 
f Grafton Elliot Smith, “ On the Morphology of the Cerebral Commissures in the Vertebrata,” etc., Trans. Linn. 
Soc. London, Ser. 2, Zoology, vol. viii, 1902, pt. xii. 
