ANATOMY OF A CYCLOSTOME BRAIN 661 



pass directly dorsalward, through and behind the primordium 

 hippocampi. The neurones of the eminentia thalami of our de- 

 scription of Ichthyomyzon are differentiated chiefly in relation 

 with this system of stria medullaris fibers. This structure is 

 well seen in our figure 4 and in Johnston's figures of Lampetra 

 ('12, fig. 5, em.th.). 



The dorsal group of stria medullaris fibers passes through the 

 substance of the primordium hippocampi, chiefly in the tract 

 which we have termed the fimbria, and related with these fibers 

 is the more dorsal eminence under the habenula which we term 

 the eminentia fimbriae (figs. 3, 4, 7, em.f.). This structure is 

 seen in Johnston's figures of Ichthyomyzon ('12, figs. 6, 9, em.th.). 

 Whether these two eminences are of common physiological type 

 has not been determined. If the primary functional connection 

 of the eminentia thalami is with the stria medullaris, as seems 

 probable, its form and topographic relations wdth other cerebral 

 structures will necessarily vary with the course of those fibers. 

 Accordingly, it cannot be used as a fixed landmark in morpholog- 

 ical interpretations of other unrelated parts of the brain. 



Our examination of the internal structure of this cyclostome 

 brain has led us to conclude that the superficial form of the ex- 

 ternal and ventricular surfaces is the expression of focal differ- 

 entiations which are functionally determined. Just how far these 

 superficial landmarks as thus defined coincide with primitive 

 morphological factors, such as metamerism, and so forth, can be 

 determined only by further embryological and comparative 

 studies. Probably the most fundamental landmark in the ver- 

 tebrate brain is the sulcus limitans of His, and it has evidently 

 been functionally determined. 



In that portion of the cyclostome brain which Ues in front of 

 the isthmus the neurones tend to be arranged in a tolerably com- 

 pact layer of cells (central gray matter) on each side of the ven- 

 tricle. In different regions under the influence of diverse func- 

 tional connections this layer breaks up into special laminae of 

 cells, which are often structurally differentiated from their neigh- 

 bors. The next step in this differentiation is the migration of 

 neurones from these laminae of central gray to form more super-. 



