MIDBRAIN AND THALAMUS OF NECTURUS 243 



of the frog. That they are actually optic fibers entering the 

 peripheral optic nerve is not definitely shown by our prepara- 

 tions, although this is certainly the appearance. At their 

 decussation the fibers of the optic tracts are in intimate asso- 

 ciation with those of the tractus tecto-thalamicus et hypothal- 

 amicus anterior, most of whose fibers are also unmyelinated 

 (see p. 256). In our material it is impossible to be certain that 

 the fibers of the so-called basal optic bundle do not come from 

 the opposite tectum opticum after decussation in the postoptic 

 commissure. 



The marginal bundle of the optic tract receives the largest 

 part of the peripheral optic fibers, these fibers crossing in the 

 most ventral and rostral part of the chiasma ridge. After 

 their decussation they pass directly lateralward and dorsalward 

 across the lateral aspect of the preoptic nucleus and thalamus 

 to enter the rostral end of the roof of the midbrain at its dorso- 

 medial border. Throughout the whole of their course within 

 the brain these fibers are strictly superficial (figs. 1 to 12, 16, 

 17, 25, 28, 36 to 41, 44, 45, 46, 48, 52 to 55, 57, 58, 65, tr.op.). 

 The part of the tectum mesencephali reached by these optic 

 fibers will be termed the colliculus superior, though there are 

 no external evidences of its precise limits. 



In the frog Gaupp ('99, p. 79) describes in the caudal two- 

 thirds of the thalamus a division of the stratum album into a 

 stratum medullare profundum and a stratum medullare superfi- 

 ciale with a gray mass, the corpus geniculatum, between. The 

 stratum medullare superficial is composed chiefly of fibers 

 of the optic tracts. Correlated wdth the reduction of the optic 

 tracts in Necturus, there is no obvious division of the stratum 

 album into deep and superficial fiber layers, nor is the corpus 

 geniculatum thalami of Gaupp especially differentiated. 



In the anterior third of the thalamus of the frog, in addition 

 to the corpus geniculatum thalami and more dorsally and su- 

 perficially of it, is a smaller circumscribed area of neuropil in 

 the stratum album, the nucleus anterior superior corporis genicu- 

 lati thalami of Bellonci ('88, p. 10). This area of neuropil 

 is well differentiated in Necturus (figs. 1, 2, p.o.th.). As the 



