ANATOMY OF A CYCLOSTOME BRAIN 639 



sis. The posterior third of this sac is entirely free from con- 

 tact with the glandular part of the hypophysis and probably 

 is comparable with the free saccus vasculosus of some teleos- 

 tean fishes and urodele amphibians, as has been pointed out by 

 Schilling ('07). 



The glandular part of the hypophysis spreads over the anterior 

 two-thirds of the infundibular sac and forward to the preoptic 

 recess, the rostral end of the mass being thicker than the caudal 

 end. It is closely applied to the nervous wall except for a short 

 distance in the groove under the preinfundibular commissure. 

 Its swollen anterior end projects forward beyond the preoptic 

 recess, from which, however, it is separated by a deep sulcus. 



The most conspicuous difference between the right and left 

 halves of the brain is due to the asymmetry of the habenulae and 

 the parts immediately related thereto. The right habenula is 

 several times larger than the left; it does not, however, overreach 

 the mid-plane, as it does in the 120 mm. specimen figured by 

 Johnston. 



The right habenula covers dorsally the entire thalamus, ex- 

 tending from the pineal recess in front of the posterior commissure 

 forward to the primordium hippocampi, whose posterior end is, 

 in fact, overlapped by the free anterior tip of the habenula within 

 the dorsal sac (fig. 4). The habenula is bounded below on the 

 lateral aspect by a sharp sulcus subhabenularis. This sulcus 

 appears on the medial surface also. 



The anterior third of the habenula is intra-ventricular, being 

 enveloped laterally by the membranous dorsal sac, whose line 

 of attachment (taenia thalami) is seen in figure 1. There is a 

 small sharply defined eminence under the free anterior tip of the 

 habenula, which is visible from both the lateral and the ventricu- 

 lar surfaces. It lies below the taenia thalami and is continued 

 backward into the substance of the habenula and forward into 

 the fimbria. It is full of small cells similar to those of the haben- 

 ula and of fibers of the fimbria system from the primordium hip- 

 pocampi to the habenula. This may be termed the eminentia 

 fimbriae (figs. 2, 3, em.f.) and appears to be merely a portion of 

 the habenula which has been differentiated in connection with the 



