180 THE BRAIN OF THE TIGER SALAMANDER 



cephalicus posterior as the precursor of the sulcus isthmi. In coil 

 ('37, fig. 1), S-reaction ('37, figs. 7, 8), and early swimming stages 

 ('37, fig. 2) the external fissura isthmi lies a little farther spinalward 

 than the internal sulcus. In slightly older early swimmers ('38, figs. 

 15-18) both the isthmic fissure and the isthmic sulcus are deep, and 

 the sulcus is displaced farther rostrally of the fissure. This means 

 that the gray of the isthmus is proliferating rapidly and pushing the 

 isthmic sulcus farther forward. The sulcus is now a very deep cleft 

 and lies approximately transversely to the long axis of the rhomben- 

 cephalon. From this stage the enlargement of the isthmic tegmentum 

 continues on the ventricular side, and in the early feeding stage (Har- 

 rison's stage 46) the isthmic sulcus extends from the cerebellum almost 

 horizontally forward before dipping downward to the fovea isthmi 

 ('386, figs. 1, 2). The sulcus and the fissure are still deep, and the 

 sulcus has come to lie far rostrad of the fissure. This is not evident 

 in transverse sections but is shown clearly in horizontal sections of 

 this and subsequent stages ('396, fig. 16). In later larval stages, in- 

 ternal differentiation, particularly the elaboration of the neuropil and 

 thickening of the auricle, result in smoother contours both internal- 

 ly and externally, and the isthmic sulcus and fissure are shallower 

 in the adult. 



At the close of the early swimming stage (about Harrison's stage 

 38) the cerebellum and auricle are so little developed that the plica 

 rhombo-mesencephalica is marked externally by a wide depression 

 ('38, fig. 17). Between this and the early feeding stage (Harrison's 

 stage 46) the sharp cerebral flexures are straightened, resulting in a 

 backward thrust of the plica rhombo-mesencephalica in the dorsal 

 wall, while the ventral wall of the isthmus remains fixed (compare 

 '38, fig. 18, with '386, fig. 1). Though there has not yet been any very 

 great enlargement of the isthmic tegmentum, the isthmic sulcus in 

 early feeders lies more nearly horizontally than vertically. In mid- 

 larval stages the deep sulcus isthmi (shown but riot named in fig. 2 of 

 '14a) is inclined more than 45° from the vertical, and between mid- 

 larval and adult stages there is enormous enlargement of the gray of 

 the isthmic tegmentum, which thrusts the sulcus isthmi forward and 

 upward toward the horizontal position. 



The external fissura isthmi does not follow the sulcus isthmi in 

 these shifts of position, but, as mentioned above, it lies in the adult a 

 considerable distance spinalward of the sulcus. That this involves a 

 mechanical stretching of the wall between the fissure and the sulcus 



