MORPHOLOGY OF THE FOREBRAIN 471 



A study of Sterzi's description ('07) of the development of 

 allied species of Petromyzon shows that the locus of the velum 

 transversum is at or near the point marked v in fig. 73. Adopting 

 Johnston's definition ('09) of the di-telencephalic boundary, fig. 

 78 lies very near the plane of this boundary, the postoptic recess 

 extending somewhat farther forward and the evaginated hemi- 

 spheres farther backward. 



The hypothalamus is separated from the pars ventralis thalami 

 by the sulcus ventralis and the large chiasma ridge. The nucleus 

 preopticus is very large and it extends forward above the chiasma 

 ridge to the lamina terminalis in relations practically identical 

 with those of the ventro-median olfactory nucleus of the hypo- 

 thetical primitive type (fig. 72, n.olf.v.m.). There is no sulcus 

 separating it from the overlying pars ventralis thalami and the 

 pars ventro-lateralis hemisphaerii ; this boundary, however, can 

 be determined by the internal structure (figs. 77, 78). None 

 of this column is evaginated into the hemisphere. The pars 

 ventro-medialis hemisphaerii, therefore, does not exist as such. 



The hemispheric evagination is composed of the whole of the 

 primary olfactory nucleus (bulbus olfactorius) and of a part of 

 the secondary nucleus; viz., of parts which correspond rather 

 closely with the regions marked and sec. olf. in fig. 72. The 

 primary nucleus forms the rostral part of the hemisphere and the 

 secondary nucleus (area olfactoria of Johnston) the caudal part, 

 the two being separated by a deep sulcus. The evagination of 

 the hemispheres is strictly lateral ; they extend a very short dis- 

 tance rostral to the lamina terminalis, but somewhat farther 

 caudad. The backward movement of the secondary olfactory 

 nucleus causes it to overlap the pars ventralis thalami as seen 

 figs. 78 and 79. The motor correlation tissue represented by the 

 latter part has advanced out a very short distance into the telen- 

 cephalon, and its relations to the interventricular foramen and 

 the other parts are essentially similar to those of the eminentia 

 thalami and corpus striatum of young urodele larvae. 



The sulcus medius extends forward to the interventricular 

 foramen. The foramen lies some distance caudad of the lamina 

 terminalis, from which it is separated by the rostral end of the 



