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foramen. Such a groove is present in embryos of all vertebrates. 
The second groove runs from the dorsal margin of the brain wall at 
the point of attachment of the velum transversum ventro-rostrally 
and, bending rostrad, continues forward to mark the boundary between 
the everted lateral lobe and the olfactory bulb. The second groove 
I have called the sulcus limitans hippocampi. It corresponds to parts 
of the grooves called sulcus limitans lateralis (hippocampi) and sulcus 
Monroi in the selachian forebrain (JoHNSTON 1911 a, p.8, 9, 42). In 
the brain wall is found a cell-free zona limitans extending from the 
p.h 
Fig. 2. Transverse section of the forebrain of an Amia of 15 mm. tr.olf.lat. 
lateral bundle of olfactory tract entering the primordium hippocampi. z.l. position of 
zona limitans. From JOHNSTON 1911b. — 
sulcus limitans hippocampi to the lateral surface a little below the 
line of attachment of the tela chorioidea (Figs. 2, 3 and 4). On the 
lateral or ventro-lateral surface in adult fishes this zona limitans is 
marked by a furrow which varies greatly in depth. This has been 
called by Kapprrs the fovea endorhinalis. I have used this term in 
a preliminary communication (1910 c, p. 154) merely for purposes of 
identification. In using it I had no intention of indicating my accept- 
ance of the homology implied in that name. In the complete paper 
now in press I have disavowed that homology and pending a later 
discussion of its significance, I have named this furrow simply the 
sulcus externus. The point to which I wish to draw attention here 
