MEDULLA OBLONGATA OF AMBLYSTOMA 379 



and fasciculus lateralis telencephali (lateral forebrain bundle, 

 tr.lat.t.). This subcerebellar eminence is separated from the 

 motor tegmentum in front of the isthmus by a sharp sulcus (fig. 

 2) which runs obliquely forward and downward from the recessus 

 posterior mesencephali. In younger stages this sulcus is shallower 

 and more nearly transverse, and in the adult it is almost entirely 

 obliterated. The eminence itself appears to have resulted from a 

 cellular proliferation in the area of discharge of the prosencephalic 

 tracts mentioned above and to serve as a distribution area for 

 these descending impulses to the motor tegmentum of the lower 

 levels of the oblongata. 



The most highly specialized neurones of the urodele oblongata 

 are sparsely scattered in the marginal zone of the gray substance 

 with their chief dendrites spread widely among the arcuate fibers 

 which pass through this zone. These cells I term collectively for 

 descriptive purposes the tangential neurones. These tangential 

 cells are correlation neurones of various sorts and are probably 

 derived, in part at least, from the ''commissural cells" of Coghill 

 ('14, fig. 27, Covi.) in very young larvae. The development and 

 connections of these neurones merit further study. 



One pair of these tangential neurones is greatly enlarged in most 

 species of fishes and amphibians and has long been known as the 

 cells of Mauthner. The position of Mauthner's cell in the half 

 grown larva is seen in figures 12 and 54, and figure '"53 shows a 

 projection of the cell and its chief processes upon the transverse 

 plane in a 12 mm. larva. The portion of the cell here shown 

 reaches through seven sections each 5/i in thickness and thus 

 extends about 35/x in a cephalo-caudal dii'ection. Only the larger 

 processes of the cell can be followed in this preparation, but these 

 reach practically the entire cross section of the stratum album at 

 this level. The axones of Mauthner's cells decussate and descend 

 in the fasciculus longitudinalis medialis in the manner usually 

 described. In this specimen they are heavily myelinated. Mye- 

 linated fibers are also seen in the fasciculus longitudinalis medialis 

 and in some other tracts of the white substance. 



In the young larva the dorsal dendrite of Mauthner's cell 

 spreads throughout the distribution area of all of the lateral line 



