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recessus vestibuli. The auditory vesicle is thick-walled, and is ellip- 
tical in cross section; one end of the ellipse isr directed upwads and 
outwards, the other downwards and inwards. The recessus vestibuli 
joins the auditory vesicle on the side next to the brain, and a short 
distance below the upper end. The distal end of the recessus is 
dilated and thinwalled, but it becomes constricted and forms a hol- 
low stalk as it approaches its connection with the auditory vesicle. 
These two parts may be distinguished as saccus endolymphaticus 
and ductus endolymphaticus. The recessus vestibuli is present in 
all the stages, but it becomes much narrower, and a distinction 
between saccus endolymphaticus and ductus endolymphaticus can 
hardly be discerned. In the oldest embryo of stage A the auditory 
vesicle has become pear-shaped in cross section, the small end being 
directed upwards and outwards. Near the upper end the lateral walls 
slightly approach each other, this being the first step in the formation 
of a semicircular canal. In stage B the auditory vesicle has become 
completely differentiated into a sacculus and a semicircular canal. 
This has been effected by the fusion and subsequent absorption 
of a portion of the lateral walls. The semicircular canal is much 
smaller in diameter than the sacculus. The recessus vestibuli retains 
its connection with the sacculus. The mesoblast surrounding the ear 
has become differentiated into a compact auditory capsule. In stage 
C the semicircular canal and sacculus are about equal in diameter, 
and the whole forms a ring-shaped canal as in the adult. 
In no other vertebrate is the ear so simple as in the Myxinoids, 
and the question arises, is this simplicity due to degeneration, or does 
the ear represent an early phylogenetic stage? I will only say that 
the facts of ontogeny, so far as known to me, do not indicate degene- 
ration. I find no structure, such as a second semicircular canal for 
example, which is present in an early embryonic stage, and which 
subsequently disappears. 
The cranial nerves present are the same that have been described 
for the adult; namely: the olfactory, the optic, the trigeminus, the 
facial, the acustic, and the vagus. There is no connection between 
ganglia and external epiblast, but this is not surprising when we take 
into consideration the advanced stage of development of the nerves. 
The optic nerve has already been mentioned, and in regard to the 
olfactory I will only say that I have been unable to find traces of it 
in either stages A or B, while in C the relations are already the 
same as in the adult. 
The remaining nerves are arranged in three groups; the tri- 
