NOTE ON THE DEVELOPMENT OF AMPHIBIANS. 321 
and uniting continuously in the region of the primitive trans- 
verse thickening. 
A subsequent development of nerve-fibres gives rise to a con- 
tinuous ventral commissure extending through the floor of the 
mid-brain and hind-brain and spinal cord ; and to the anterior 
and posterior commissures of the brain. 
The fibres of the optic nerves are intimately connected with 
and are developed in the same manner as the main bundle 
of fibres in the region of the primitive transverse epiblastic 
thickening. 
The hypophysis of Amblystoma presents a form of develop- 
ment intermediate to that of the Lizard and that of the Frog. 
The balancers of Amblystoma may be considered as external 
gills of the mandibular arch which have become metamor- 
phosed into embryonic organs of support. 
P.S. — In his work entitled ‘ Untersuchungen fiber die ver- 
gleichende Anatomie des Gehirns/ Dr. Ludwig Edinger has 
described a Commissur der basalen Vorderhirnbundel, 
which he says appears in all classes of Vertebrates. The 
position of this Commissur in the adult brain immediately 
behind the optic chiasma is identical with that of the anterior 
band of nerve-fibres (A. F.), which I have described in the 
embryonic condition. The relatively large size and pronounced 
character of the anterior band in both Reptilian and Amphi- 
bian embryos lead me to think that it was once of primary 
importance, and that the Commissur in the adult brain is 
probably a rudiment of the same with changed relations and 
functions. 
