1911] Chandler: A Lymphoid Structure in Lepisosteus 95 
ings are larger and much more pronounced, curving forwards 
or backwards, so that in some sections they appear entirely sep- 
arated from the ependyma, and look lke cavities lined with 
epithelium and surrounded by the reticular connective tissue 
lying over the myelencephalon. Farther caudad than the region 
represented in plate 12, figure 4, also, the cavity over the brain 
is much extended laterally, and the lateral portions are largely 
filled in with embryonic connective tissue cells. These regions 
seem to be the centers of proliferation of the connective tissue, 
as here the cells are hardly differentiated, while towards the 
median line the reticular processes of the cells become more and 
more developed, and the cells themselves become fewer in number. 
At this stage the gland has not the definite outline which is 
apparent in the adult, and there is still no indication of the 
leucocytes or granular masses which form such a conspicuous 
part of the fully developed structure. 
It has not been possible thus far to obtain a specimen inter- 
mediate between this 55 mm. stage, and a young adult of 250 
mm., the brain of which is figured entire in figure A. This spee- 
imen has not yet been sectioned, but as it is adult in all characters 
except size, it is doubtful whether it will throw any more light 
on the development of the gland. It has, in this specimen, the 
characteristic shape, but is shghtly smaller in proportion to the 
brain than in older individuals. 
SUMMARY 
1. A lymphoid, gland-hke structure overlies the myelencepha- 
lon of Lepisosteus, and is closely associated with the pial covering 
of the fourth ventricle. Though large and conspicuous, it has 
been almost universally overlooked by workers on the brain of 
Lepisosteus. 
2. The myelencephalic gland (provisionally so named) 1s 
present in both Lepisosteus osseus and L. platystomus, which are 
the only species of the genus examined by me. No indication 
of it has been found in other ganoids. 
3. The size and form are very constant in all specimens dis- 
sected and are practically the same in both species. Topograph- 
ically it seems to be a highly developed portion of the pia mater. 
