392 NILS HOLMGREN 
the anterior and the posterior horns (if present) of the lateral 
ventricles. The anterior ventricle extends into the olfactory bulb 
and forms the ventricle of the bulb. Through the inversion the 
dorsal parts of the embryonic lateral plates of the forebrain rudi- 
ment (‘Anlage’) not only are brought in contact with each other, 
but also are sometimes bent down parallel with each other to form 
in the roof an incomplete septum, dorsal to the foramen monroi, 
between the lateral ventricles. Through the anterior evagina- 
tion, parts of the brain vesicle, which hitherto were situated on 
the lateral side, are brought into medial position in front of the 
lamina terminalis to form a forward continuation of the imper- 
fect dorsal septum mentioned above. The anterior septum will 
thus be complete. As there is in the forebrain vesicle a roof- 
and a floor-part, the perfect septum must be composed of cor- 
responding parts: the roof-part is the medial part of the archi- 
pallium (if differentiated), the floor-part is the paraterminal 
body or septum pellucidum of the authors. The limit be- 
tween the floor-part and the roof-part is in many cases a very 
distinct one, a zona limitans (in teleosts, selachians, ganoids, 
amphibians, reptiles, birds, and perhaps also in mammals). It 
seems to be very unnecessary to make these statements made 
several times before, but in a recent paper Lundahl has de- 
clared that the septum pellucidum (paraterminal body) is a pal- 
lial structure, that he names palaeopallium. This curious view, 
which is not consistent with well-known facts, seems to fall to the 
ground in view of the above statements concerning the evagina- 
tion of the hemispheres. 
As to the everted brain type, it may be accentuated that 
this is surely not primary in character, but rather to be derived 
from an inverted and evaginated type. This seems to be evident 
a priori from the fact that the brain of the lower vertebrates 
(cyclosomes, selachians) is inverted; and also in the organiza- 
tion of the everted brain there are present the most distinct signs 
of its origin from an inverted type. I will call attenton at once 
to the fact that in ganoids and certain bony fishes (Lophius, 
Anguilla) there is a ventricle in the olfactory bulb. This ven- 
tricle is continued backwards by a groove which caudad decreases 
