The brain of Acipenser. 119 
thalamicus through the thalamus to the epistriatum. The remainder 
form a bundle which runs at first dorsad and then turns caudad 
through the commissura ansulata into the base of the medulla. 
These are paired bundles near the mid-ventral line and ventral to 
all the tracts from the lobi inferiores and tectum to the medulla, 
tractus mammillo-bulbaris. 
Fibres enter the corpus mammillare from two sources. It shares 
in the distribution of the fibres of the secondary olfactory tracts 
which enter the hypothalamus, and in addition receives the bundle 
from the nucleus anterior as noted above (page 113). 
Latero-dorsal to the corpus mammillare, at the junction of the 
latter with the thalamus, is a diffuse nucleus of cells somewhat 
larger than those of the corpus mammillare, measuring 8—23 by 
16—26 u. The cells are fusiform or stellate and have much thicker 
dendrites than those of the corpus mammillare cells. I have been 
unable to ascertain with full certainty the source of fibres entering 
this nucleus or the destination of its neurites, the nucleus not being 
well impregnated in most of my preparations. Fibres from the 
nucleus anterior seem to end here and the neurites go into the 
tractus strio-thalamicus medius, probably to go to the epistriatum. 
From its position I shall designate this nucleus the corpus ecto- 
mammillare. 
4. The Pituitary Body. 
I use the name pituitary body to indicate the whole ventral 
appendage of the 'tween brain, saccus vasculosus for the outgrowth 
from the ventral wall of the ‘tween brain, and hypophysis for the 
body which is formed by invagination from the ectoderm (DOHRN, ’83; 
von KUPFFER, 93, and LUNDBORG, ’94). These structures are best 
studied in sagittal sections stained with methylene blue and acid 
fuchsin. The saccus is very large in Acipenser and has a large 
cavity which communicates by a wide passage with the cavity of 
the corpus mammillare (Phots. 3, 4, 18). The common cavity of the 
saccus is produced laterally and caudally into numerous branching 
canals or pouches. The wall consists of the following layers from 
within outward: a lining epithelium continuous with the epithelium 
lining the brain cavities, a layer of nerve fibres, a basement mem- 
brane, and a thick layer of connective tissue in which are imbedded 
enormous blood or lymph spaces. The connective tissue is actually 
small in amount and the blood spaces with their contained corpuscles 
