The nucleus of BELLONCI in birds - DWC

1100
upper part of a cell plate, which for the rest gives rise to the ventral lateral
geniculate 1) , and thus would belong to the ventral thalamus. According
to KUHLENBECK (1937), working also with the chick, the nucleus lateralis
anterior, however, is a rostral differentiation of the primordium of the
dorsal lateral genieulate, which belongs to the dorsaI' thalamus.
In amphibians as well as in reptiles and mamma Is the nucleus of
BELLONCI is an ellipsoid, whose major axis surpasses the langer of the
minor axes two times or more in length. In the first class the major axis
of the ellipsoid is directed sagittaIly, whereas in reptiles and mammals it
is situated in a transverse plane. In the previous paper I explained th is
difference by a turning of the nucleus through 90°. It may as well be,
however, that it has been brought about by a compression in sagittal
direction. Like the nucleus of BELLONCI also the nucleus lateralis anterior
is an ellipsoid, with, however, but a slight difference in length between the
major axis and the langer of the minor axes.
As to the histologieal structure of the nucleus of BELLONCI, in amphibians
this nucleus is a dense ma ss of neuropil, with but a few cells within it,
almost all of these being ~ituated outside the neuropil, viz., on its medial
side. In reptiles and mammaIs, however, the cells lie within the neuropiI.
The avian nucleus lateralis anterior is likewise a dense ma ss of neuropil.
with the cells within it.
A comparison of the connections of the two nuclei also strongly speaks
in favour of their homology. In the previous paper it was set forth that the
chief supply to the nucleus of BELLONCI (in amphibians and crocodilians
at least) was by the tractus olfacto~habenularis Jateralis 2) , and that a
contribution from the optie might be deemed certain in amphibians,
prdbable in reptiles, and not excluded in mam mals. The connection with
the optic was confirmed of late by STRÖER (I 939) , who found that in
specimens of Triturus taeniatus wh ere in early stages the primordium of
one eye was extirpated, the neuropil of BELLONCI on the heterolateral side
was smaller than on the other.
As to the connections of the nucleus lateralis anterior, when describing
this nucleus in the sparrow and dove, lalready expressed as my opinion
1) As set forth below. the nucleus lateralis anterior is identical with RENDAHL's
nucleus fasciculi septi (labelled ä by him). while- the ventral lateral geniculate of
KUHLENBECK and us corresponds with RFNDAHL's corpus geniculatum thalamicum.
2) In the previous paper I distinguished in crocodilians a tractus olfacto-habenularis
lateralis anterior and posterior. identifying the former with the tractus olfacto-habenularis
lateralis of Miss CROSBY (1917) and the latter with her tractus olfacto-habenulari~
posterior. As I now perceive. the latter identification is not correct. The tractus oHactohabenularis
posterior of Miss CROSBY arises near the posterior end of the hemisphere
from the nucleus of the lateral olfactory tract and the ventro-medial nucleus. The fibres.
however. described by me as tractus dJfacto-habenularis lateralis posterior. from an
unknown souree ascend through the forebrain bundIe to the ~tria medullaris. They have
already been seen by UNGER (1911) (his tractus transversalis taeniae) in crocodilians
and by CAIRNEY (1926) in Sphenodon. Thus the two terms proposed by me have to be
dropped.