Houser, Zhe Neurones of a Selachian. To 
limit of the bilaterally symmetrical anlage may be definitely 
fixed, coinciding with the boundary between the primary mid- 
and hindbrain vesicles. The posterior limit of the future cere- 
bellum is by no means clear, however, for it seems to merge 
backward into the oblongata. These are certainly significant 
facts. 
It was shown in Section IV, Subsection 7, that the molec- 
ular layer of the cerebellum is continuous with the cerebellar 
crest of the tuberculum acusticum, maintaining essentially the 
same morphological characters throughout. It was also shown 
that there are present in the tuberculum acusticum neurones 
which are identical with those of the cerebellum—molecular 
neurones, granular neurones, and PURKINJE neurones; the last 
two varieties are not grouped into definite strata, however. The 
presence of granular neurones in the acusticum is worthy of 
remark, but the greatest weight must be attached to the find- 
ing of PURKINJE neurones here. These neurones were long 
supposed to characterize the cerebellum alone, and their strik- 
ingly peculiar appearance makes them readily identified. The 
neurones of PurRKINJE from the acusticum of Mustelus agree 
with those from the cerebellum as to size, general form, shape 
and character of dendritic top, and even in the presence of the 
spiny gemmules so characteristic of this nervous element. There 
can be no question as to their morphological identity in Mus- 
telus. JoHNSTON ('98b) has found PuRKINJE neurones in the 
acusticum of Acipenser, somewhat smaller and simpler than 
those of the cerebellum, but undoubtedly equivalent. Similar 
results will probably be obtained in all of the simpler ver- 
tebrates. 
It may be concluded, in the light of these embryological 
and structural facts, that the cerebellum has arisen in the phylo- 
geny of vertebrates as a fused outgrowth of the pair of tubercula 
acustica, The acusticum is the primary end-station, as we 
have seen, for the nerves of the ear and the lateral line organs. 
The cerebellum has been differentiated from the primary ending 
as a special centre for presiding over equilibration. Parallel 
with the increasing development of the equilibrial sense in ver- 
