WM) Chandler: A Lymphoid Structure in Lepisosteus 87 
walled ‘‘ vesicle of the thalamencephalon,’’ which had previously 
escaped notice and is readily destroyed in dissection, yet they 
make no reference to any gland-like structure lying over the 
myelencephalon, and the usual opening into the fourth ventricle 
is Shown in their figure. They also made sections of the entire 
head of embryos up to the 26 mm. stage in working out the 
development of the brain, but, since the gland is not reeogniz- 
able, as such, up to that stage, it escaped their notice again. 
In tearing off the gland in a dissection of the brain, it is 
impossible not to tear off the posterior medullary velum with 
it, which accounts for the foramen of Magendie usually shown 
in figures of the brain of Lepisosteus, as seen in the figures of 
Busch (1848), Mayer (1864), Owen (1868, vol. 1, fig. 174), 
Huxley (1872, fig. 38), Wilder (1875, pl. 2, fig. 7), Parker and 
Balfour (1882, pl. 25, fig. 478), and Allen (1907, pl. 6, figs. 11 
and 12). Kingsbury (1897, pl. 6, fig. 5) has a diagrammatic 
sketeh of a cross-section of the myelencephalon in which he shows 
the roof of the fourth ventricle undisturbed, but without the 
gland. 
OCCURRENCE AND HOMOLOGY 
The possibility that this gland might be an abnormal develop- 
ment presented itself, and to determine this point two other speci- 
mens of Lepisosteus osseus were dissected. The same structure 
was found in each of them, and was of the same size and shape. 
It was also found in Lepisosteus platystomus, only one specimen 
of which was available. No example of L. tristaechus, or alli- 
gator gar, could be obtained for dissection, but on account of 
the similarity of this species to L. platystomus in other respects. 
and also because of the very close similarity of the gland in the 
two more divergent species examined, it is probable that this 
organ will be found in the third species. 
It was thought that some such development would be found 
in other types of ganoid fishes, and in the hope of finding some- 
thing at least suggestive of it, numerous specimens of ganoid 
brains in the collection of Dr. Wilder were examined, including 
examples of Acipenser, Scaphirhynchus, Polyodon, and Amaia. 
In none of these, however, could any indication of it be found, 
