THE TELENCEPHALON IN CYCLOSTOMES 367 
SUMMARY 
The evagination of the hemispheres is at a low stage in petro- 
myzonts. In Ichthyomyzon and Lampetra the formatio bulbaris 
is not all evaginated. The fact that a part of the wall of the third 
(median) ventricle in these forms is made up of formatio bulbaris 
ought to be convincing evidence, if any further evidence is needed, 
that a part of the third ventricle belongs to the telencephalon. A 
large part of the secondary olfactory centers is evaginated, but 
a relatively much larger portion of these centers remains in the 
telencephalon medium than in the case of selachians. The prim- 
ordium hippocampi remains wholly in the telencephalon medium. 
The exact relations of the somatic area await further study. 
The di-telencephalic boundary is marked by the attachement 
of the velum transversum to the eminentia thalami as in selachians 
ganoids and amphibians. The eminentia thalami is a well-marked 
ridge just rostral to the nucleus habenulae, bounded by the sulcus 
medius and by the sulcus imitans hippocampi, and traversed by 
the stria medullaris. 
The telencephalon medium, which is relatively larger than in 
higher vertebrates, stands as a wedge-shaped mass between the 
hemispheres. Its massive nervous walls are connected with one 
another by the lamina terminalis, lamina supraneuroporica and 
tela chorioidea. The membranous roof plate (His) in the telen- 
cephalon of petromyzonts connects the two simple unevaginated 
lateral walls of the neural tube as in any other segment. The 
hemispheres are evaginations of restricted areas of these lateral 
walls. The foramen is roofed over by the massive primordium 
hippocampi and there is no extension of the tela chorioidea into 
the roof of the hemispheres. 
The primordium hippocampi is represented by the ‘praethala- 
mus’ of previous authors (the epistriatum of my earlier paper, ’02 
a). Itisalarge body having a highly developed and characteristic 
finer structure which already presents most of the relations of the 
primordium hippocampi of higher forms (see text). The two 
primordia converge forward to form a bed for the anterior pallial 
commissure in the lamina supraneuroporica. The primordium 
is separated from the epithalamus, thalamus, hemisphere and the 
