MEDULLA OBLONGATA OF AMBLYSTOMA 353 



shows a distinct ventricular swelling at the levels of the super- 

 ficial origins of the VII and VIII nerves, and also an anterior lobe 

 which forms the lateral wall of the recessus lateralis. 



The body of the cerebellum is larger in this larva than in adult 

 Necturus and it occupies the entire rostral end of the auricular 

 lobe and here root fibers of the cranial nerves extend forward from 

 the area acustico-lateralis into the body of the cerebellum, a 

 condition which has not been demonstrated in Necturus. The 

 roof of the wide recessus lateralis is entirely membranous and 

 plexiform. No massive cerebellar tissue is developed in the mid- 

 dorsal plane except the fibers of the decussatio veli, which has the 

 same components as I have already described ('14) for Necturus. 

 Further details regarding the development and structure of the 

 cerebellum of Amblystoma are reserved for a later communication. 



The isthmus rhombencephali is not well developed ventrally, the 

 massive fiber tracts of the pedunculus cerebri, lemniscus, etc., 

 forming a strong ventro-lateral ridge between the rhomben- 

 cephalon and mesencephalon. But dorsally the isthmus is marked 

 by a deep total fold of the brain wall within which lies the IV nerve 

 (figs. 1 and 2). The caudal lip of this fold is formed by the body 

 of the cerebellum; the rostral lip by the tectum mesencephali. 



The commissura tecti is feebly developed except for the massive 

 posterior commissure at its rostral end. The caudo-lateral aspect 

 of the tectum shows a well defined eminence, the nucleus posterior 

 tecti (fig. 1, nuc.p.t.), and on the ventricular surface there is a 

 corresponding evagination of the optocoele, the recessus posterior 

 mesencephali (fig. 2, r.p.m.). The tectum mesencephali is 

 bounded ventrally on the ventricular surface by a very shallow 

 depression, which probably represents the position of the sulcus 

 limitans, and below this is the motor tegmentum. The latter 

 consists of two very distinct parts separated by a narrow and 

 very deep sulcus which extends forward and downward from the 

 ventral end of the recessus posterior mesencephali. Below this 

 is the massive eminentia subcerebellaris tegmenti (fig. 2, em.s.t.). 

 The ventral commissure system is well developed throughout the 

 entire extent of the midbrain and oblongata, and is evidently very 

 complex. 



