470 C. JUDSON HERRICK 



sulcus limitans (sulcus Monroi of Reichert) is a landmark of prime 

 importance embryologically and Mihalkovics ('77, p. 70) in his 

 discussion of the relation of the diencephalon to the hemispheres 

 seems to include both my sulcus medius extending back from the 

 interventricular foramen and the sulcus limitans extending back 

 from the preoptic recess in his sulcus Monroi. His applies the 

 latter term to the rostral end of the sulcus limitans only. Mihal- 

 kovics clearly describes the forward extension of the ventral 

 lamina into the lateral wall of the hemisphere j but neither he nor 

 his followers seem to have fully appreciated the significance of 

 the direct passage of the great olfactory radiation into the hypo- 

 thalamus and the morphological significance of the latter as a 

 motor correlation center derived from the ventral lamina of the 

 neural tube. 



In the cyclostome brain the optic vesicles have been completely 

 evaginated and the evagination of the cerebral hemisphere has 

 advanced scarcely beyond the first step, viz., the outgrowth of 

 primary and secondary olfactory centers. 



Through the kindness of Dr. Charles Brookover I have been 

 able to study a carefully prepared series of transverse sections 

 through the head of a 120 mm. specimen of Ichthyomyzon con- 

 color (Kirtland) stained with haematoxylin, and I present here- 

 with (figs. 73 to 81) a series of sketches from these preparations 

 to illustrate the relations of the diencephalon and telencephalon. 

 This material by itself is of course insufficient for such an analysis, 

 but the descriptions of Sterzi, Johnston and others enable us to 

 supplement these observations and to present the following inter- 

 pretation. 



The sulcus limitans is not clearly preserved in the diencephalon 

 of this specimen. In the caudal part of the diencephalon (fig. 

 81) the dorsal (subhabenular,) medial and ventral diencephalic 

 sulci are seen substantially in the same relations as in the Amphi- 

 bia. The ventral sulcus is interrupted by the chiasma ridge 

 (figs. 80 and 73), but the medial sulcus continues rostrad as far 

 as the interventricular foramen (figs. 79, 78, 77). The relations 

 of the diencephalon and telencephalon as seen in a reconstruction 

 of the median section are shown in fig. 73. 



