No. I.] AMPHIBIAN BRAIN STUDIES. 57 
and cerebellum of the former. The cerebellum is generally 
found to have a direct relation to the size and activity of the 
limbs; it is extremely small in the limbless forms, intermedi- 
ate in the Salamanders, and so on, increasing to the Avzra. 
Are we-to consider the Urodele cerebellum.as in a 
degenerate orina primitive condition? The limbs 
of Cryptobranchus (Menopoma) are well developed and fully func- 
tional; they are, moreover, in the most primitive condition 
known among the Amphibia (Baur, ’88, p. 55). If the relation 
above mentioned holds good here, we may infer that the cere- 
bellum is also in a primitive condition in the central Uvrodela 
phylum. It may be considered in a degenerate condition in 
some of the limbless types. 
This correlation has a direct bearing not only upon the ques- 
tion of the number of the larger encephalic segments in the 
Amphibia, but upon the origin of this segment. By many au- 
thors the cerebellum is considered a distinct segment equiva- 
lent, for example, to the mid-brain. By others it is considered 
as a portion of the roof of the hind-brain. The hypothesis I 
ofer 1s that the cerebellum is primitively interseg- 
mental, and secondarily acquires a functional im- 
portamce equivalent to that of the other secments, 
(Osborn, ’87, p. 940). 
The cerebellum of C7yptobranchus (see also p. _) is chiefly 
composed of decussating tracts, passing on the one side into the 
lateral regions of the medulla, on the other into the mesenceph- 
alon. It may even be questioned whether we have here the 
essential elements of the cerebellum, the structure is so ex- 
tremely simple. It has been stated that the posterior com- 
missure, which invariably marks the dorsal boundary between 
the dien- and mesencephalon, is not a commissure in the strict 
sense of the word, but consists of fibres from the two tegmental 
tracts 1 decussating to the opposite side of the brain. Similarly, 
the superior commissure, described independently by Bel- 
lonci (81) and myself (’84, p. 268), consists of fibres passing 
across the roof of the third ventricle from the diencephalon 
to the opposite prosencephalic segment (see secs in all the fig- 
1 With special relations to the tegmental tracts (Pawlowsky) and the nucleus of 
the third nerve (Darkschewitz). 
