222 C. JUDSON HERRICK 



(figs. 11, 46, 47, 62, 63, 64, f.i.), which lies at about the trans- 

 verse level of the roots of the III nerves. Large blood vessels 

 enter the brain floor at this point. Here the ventral commissure 

 system is interrupted and the gray layer extends to the ventral 

 surface of the brain, as was noted by Kingsbury ('95, p. 163), 

 who termed this depression the mesencephalic pit. This land- 

 mark was first noted by Stieda (75, p. 294), who described it 

 in the axolotl (Amblystoma mexicanum) as a slight but evident 

 constriction of the pedunculus cerebri which marks the boundary 

 between the pars peduncularis and the medulla oblongata. 

 Burckhardt ('91, p. 379) noted its presence in Ichthyophis and 

 numerous other vertebrates, and called it the 'Mittelhirngrenze.' 

 His ('92, p. 355) again refers to it under the name 'Isthmus- 

 grube,' and this designation in its Latin form is here adopted. 



In adult Necturtis the fovea isthmi lies so far forward as to 

 raise the question whether it really marks the caudal border of 

 the pedunculus cerebri, as the authors just cited have main- 

 tained. The figures published by these authors, however, show 

 that in early developmental stages of vertebrates in general it 

 lies relatively farther caudad. 



Embryological stages of Necturus not being available, I have 

 examined the relations of this landmark in a series of wax 

 models of the brain of Amblystoma tigrinum from the 10 mm. 

 larva to the adult. In the adult the fovea is not as conspicuous 

 as in Necturus; nevertheless its position is quite e\ddent in 

 essentially the same relations when the more compact form of 

 the Amblystoma brain is taken into account. In the 10 mm. 

 larva, on account of the great mesencephalic flexure, the tu- 

 berculum posterius lies in the same transverse plane as the re- 

 cessus posterior mesencephali (see p. 224) and the posterior com- 

 missure lies far forward, so that the fibers of the descending 

 Hmb of this commissure, to reach their termination above the 

 tuberculum posterius, must take a course nearly horizontal 

 with reference to the body axis. From the fovea isthmi a 

 strong sulcus, which I shall call the sulcus isthmi (figs. 63, 64, 

 s.is.), runs dorsalward to end in the recessus posterior mesen- 

 cephali. The eminentia subcerebellaris tegmenti (see p. 225) 



