ANATOMY OF A CYCLOSTOME BRAIN 641 



Each cerebral hemisphere is divided by a deep obhquely trans- 

 verse sulcus into two unequal parts; the larger anterior part is 

 the olfactory bulb and the posterior part is the secondary olfac- 

 tory area, or lobus olfactorius (figs. 1, 2, l.oL). These parts are 

 separated from the telencephalon medium by a sharp stem- 

 hemisphere fissure except along the ventral border. Here the 

 transverse sulcus also disappears, this region being occupied by 

 a ventral eminence containing dorsally the primordium of the 

 corpus striatum and ventrally the nucleus olfactorius medialis. 



The forebrain of Ichthyomyzon is more compressed in the 

 cephalo-caudal direction than are the brains of most other pe- 

 tromyzonts which have been described, though the model of the 

 140 mm. specimen is somewhat less so than is Johnston's model 

 of the 120 mm. specimen (cf. our figure 4 with his figure 6) the 

 ratio of length to height in his model being 48 : 100 and in our 

 model 58 : 100, leaving out of consideration the dorsal sac in 

 both cases. 



MEDIAL SURFACE 



Figure 4 illustrates the ventricular surface and the cut surfaces 

 of the right half of the divided model, and figure 3 is a key dia- 

 gram to indicate the relations of the ventricular' eminences and 

 sulci and some of the underlying structures. 



The cerebral ventricle in front of the isthmus is high in dorso- 

 ventral diameter, but very narrow in transverse diameter ex- 

 cept in a few places where lateral evaginations of the whole wall 

 cause shallow or deep pockets on the ventricular surface. These 

 evaginations are as follows: (1) the tectum mesencephali, marked 

 on the ventricular surface by the optocoele; (2) the metathala- 

 mic recess; (3) the pineal recess; (4) the dorsal sac; (5) the cer- 

 ebral hemisphere; (6) the preoptic recess; (7) the postoptic re- 

 cess; (8) the lower part of the infundibulum (saccus vasculosus 

 of Johnston); (9) the mammillary recess. The remaining parts 

 of the ventricular surface show a definite sculpturing in low re- 

 lief, whose sulci do not involve the entire thickness of the brain 

 wall. 



