FOREBRAIN MORPHOLOGY 397 
lateral ventricle splits up into two parts, a rostal and a caudal 
one. The first is the ventricle of the olfactory bulb (fig. 4, 
0.v.) homologous with the same in other animals. The second 
(figs. 1, 2) has been compared with the lateral ventricle of the 
hemisphere and regarded as homologous with this. 'The common 
aperture of the ‘lateral ventricles’ into the medial ventricle has 
been regarded as the foramen monroi. This conception of the 
Petromyzon forebrain is not allowable, as will be clearly demon- 
strated by means of an examination of the ventricular system in 
an Ammocoetes of 1.7-cm. body length. In this stage the fore- 
brain has the same exterior aspect as in more advanced stages 
and in adult animals, but the ventricles are arranged in another 
way. The bulbar and the hemispheral ventricle are free from 
each other (compare figs. 4, 2, and 1), opening into the medial 
ventricle with separate openings, the bulbar lying ventral to the 
hemispheral. From the bulbar ventricle a distinct groove runs 
backwards in the wall of the medial ventricle (fig. 1, s.l.e.) and a 
good distance under the opening of the hemispheral ventricle. 
This groove apparently is the sulcus limitans externus, as pointed 
out above. ‘This furrow in other vertebrate brains is seen at the 
lateral wall of the lateral ventricle in various positions in different 
vertebrates, but always belonging to the lateral wall of the lateral 
ventricle. According to the current view of the ventricular sys- 
tem in Petromyzon, the sulcus limitans externus here should not 
belong to the lateral ventricle. It is highly improbable that 
Petromyzon should in this respect represent a quite different type 
than all other vertebrates. Thus I am obliged to accept the 
opinion that the hemispheral ventricle in Petromyzon represents 
only the dorsolateral part of an ordinary forebrain ventricle. 
This dorsolateral part is tube-like, directed towards the neck 
part of the pallium, perhaps representing a ventricle analogous 
to the posterior diverticulum of the forebrain ventricle in Acan- 
thias. Owing to this, it is clear that the foramen monroi of 
the Petromyzon brain is no true foramen monroi. The ventricu- 
lar system in the forebrain of Petromyzon thus is to be considered 
as a highly reduced one. This reduced ventricular system 
indicates that also the entire forebrain is not ancestral, but 
