THE MORPHOLOGY OF THE DIENCEPHALIC FLOOR 
A CONTRIBUTION TO THE STUDY OF CRANIATE HOMOLOGY 
FREDERICK TILNEY 
' From the Department of Anatomy, Columbia University 
THIRTY FIGURES 
The fact that the basal surface of the brain-stem is phy- 
letically more constant in form than the lateral walls or the roof 
is so obvious as scarcely to need mention; yet in some respects 
it is a fact of considerable significance. It seems to have im- 
portance in estimating the homological values of certain parts 
of the basal region since this region in consequence of its greater 
morphological constancy should afford more exact evidence of 
homology. This becomes especially clear when it is considered 
how plastic are the lateral walls of the neural tube, not only 
in their embryonic development but in the adaptive modifi- 
cations of which they are capable. A similar plasticity is seen 
in the roof-plate as witnessed by the varied conformation of 
the paraphysis, habenular region, epiphysis, mesencephalic 
tectum and cerebellum. - 
The basal region, on the other hand, is not without its vari- 
ations; nor is it surprising that this region should bear traces 
of primitive characters, particularly in the interbrain, where 
the neural structures have always maintained such intimate 
relations to the stomadaeum, pituitary gland and _ branchial 
cavity. Andriezen (1), among others, has brought this fact 
out clearly. He observed in Ammocoetes, Amphioxus and 
Balanoglossus, as well as in the larval and adult forms of Ascid- 
ians, a small tubular aqueduct of capillary lumen and lined 
by ciliated epithelium, extending between the mouth cavity 
and the forebrain. This bucco-neural duct, he believes, pro- 
vides a true water-vascular system for the central nervous 
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