FOREBRAIN MORPHOLOGY 395 
Through this sulcus the pallial parts of the brain are approximately 
separated from the subpallial. The pallium is medially bordered 
by the primordium hippocampi of Johnston (praethalamus of other 
authors, fig. 2, p.h.). I wish not to express any definite opinion 
as to this primordium hippocampi. Johnston’s opinion is 
founded on the behavior of the velum transversum, but there 
seems to be some analogy between the primordium hippocampi 
and the eminentia thalami of amphibians, especially in the rela- 
tions to the taenia bundles. Also the embryological develop- 
ment of this hippocampal formation is not convincing to the ad- 
vantage of Johnston’s interpretation. For the ‘primordium 
hippocampi’ of the Ammocoetes-larva of 30 mm. is an ependymal 
border of the medial pallium—a border that insensibly passes 
over into the ependymal roof of the brain-vesicle (fig. 2, p.h.). 
Later the primordium grows thicker (fig. 3A) as the taenia . 
bundles enter it, and it becomes ganglionic. The fact that the 
‘primordium hippocampi’ is situated before the point of lateral 
attachment of the velum transversum is not a fact of definite 
importance, since there is no ground present why the di-telen- 
cephalic border should be a straight line behind the ‘primordium 
hippocampi’ and not a bent line before this part of the brain. 
A comparison between figures 3A and 3B, respectively from Am- 
mocoetes and Triton, reveals so great concordance that I find 
the opinion of the ‘primordium hippocampi’ as a part of thalamus 
comparatively well settled. 
I thus regard as the pallium of Petromyzon what Edinger 
calls the lobus olfactorius. In the cephalic part of this lobus 
olfactorius he observed a small dorsomedial part, which he 
thought to be an ‘Episphaerium-Anlage’ (fig. 4, “Hp.’’). This 
part is a great olfactory glomerulus with dorsocaudal situation 
in the olfactory bulb. (In the figure in his ‘Vorlesungen,’ which 
illustrates the ‘Episphaerium’ rudiment, the ‘primordium hippo- 
campi’ of Johnston is falsely labeled ‘ganglion habenulae.’) 
In the pallium there is present a cortical layer of propyramidal 
cells (fig. 2, Cort. 1). This layer is somewhat diffusely composed, 
but nevertheless very conspicuous. It is laterally somewhat 
subdivided, but the subdivisions have rather the appearance of 
