ISTHMUS 181 



in later stages is evident from the arrangement of the ependymal ele- 

 ments in the lips of the isthmic sulcus. This traction begins early. It 

 was noticed in early swimmers ('38, p. 220) that the ependymal ele- 

 ments of the sulcus isthmi are thickened and sharply bent outward 

 and backward. This was illustrated also in early feeding stages ('386, 

 figs. 11, 12) and in Golgi impregnations of midlarval stages ('396, 

 p. 518, and figs. 31, 93). At all stages, including the adult, these thick 

 elements form a dense band of parallel fibers bordering the sulcus on 

 both sides and extending outward and backward to an attachment 

 at the pial surface in the floor of the isthmic fissure. In the adult the 

 sulcus isthmi usually does not extend ventrally so far as the fovea 

 isthmi, but its locus here is indicated by a band of modified ependy- 

 ma. Two mechanical factors, accordingly, can be recognized in the 

 shifting relations of the isthmic fissure and sulcus : first, the backward 

 thrust of the roof during the straightening of the cerebral flexures 

 between early swimming and early feeding stages and, second, un- 

 equal growth of superficial and deep structures, especially in later 

 stages. The rapid enlargement of the gray of the isthmic tegmentum 

 in late larval stages produces an upward and forward thrust ventral- 

 ly in the direction opposite to the dorsal backward thrust of early 

 stages. 



The developmental history and adult position of the sulcus isthmi 

 of Amblystoma are radically different from those of Hynobius as de- 

 scribed by Sumi ('26), who illustrates models showing the ventricu- 

 lar sculpturing from closure of the neural tube to the adult. At none 

 of these stages is von Kupffer's sulcus intraencephalicus posterior 

 visible at the plica rhombo-mesencephalica. The sulcus isthmi ap- 

 pears rather late at the fovea isthmi, and it does not at any stage 

 reach the dorsal surface. 



SENSORY ZONE 



The sensory field of the isthmus is contracted. Its scanty gray sub- 

 stance includes a compact cluster of cells of the mesencephalic V nu- 

 cleus within and adjacent to the anterior medullary velum. These 

 cells are smaller and more densely crowded than are those of this nu- 

 cleus in the tectum, and they may have dift'erent functional signifi- 

 cance (p. 140). Their connections are unknown. Associated with them 

 are other cells at the posterior border of the inferior colliculus which 

 appear to have peripheral connections. From this region a few mye- 

 linated, and many unmyelinated, fibers go out to the meninges and 

 chorioid plexus of the fourth ventricle and some with the IV nerve. 



