1911} Chandler: A Lymphoid Structure in Lepisosteus 9] 
medulla, however, the tissue of the gland is continued into it 
as shown in the region marked ‘‘A’’ in plate 10. As stated 
above, there is no apparent differentiated covering of the eland. 
Of the various histological elements present, the most con- 
spicuous are the extremely large and irregular, black pigment 
cells, scattered irregularly throughout the whole organ. These 
cells are similar to those in the pigmented covering of the brain, 
except that they are not so flattened, and send their branches 
freely in all directions. Under the high power of the microscope, 
they show the typical granular structure of melanin pigment 
cells, and the granules are often somewhat scattered at the peri- 
phery, where the cell has been cut. Some of these pigment cells 
measure fully eighty microns from tip to tip of their branches. 
In the middle of the dorsal part of the gland there is an open 
reticulum of connective tissue which is gradually encroached 
upon by the more solid substance (pl. 10, fig. 1). From the 
microscopical appearance of other parts of the gland, there is 
much evidence that this reticulum forms the framework for the 
entire structure, the other elements being netted in it. The 
‘ 
appearance of this network in a ‘‘solid’’ part of the gland may 
be seen in plate 11, figure 2, which represents such a portion 
highly magnified. Farther cephalad than the region shown in 
plate 10, figure 1, which represents a section slightly in front of 
the middle, the network becomes more and more open, until, 
on the sides under the cerebellum, nothing is left but the retie- 
ular connective tissue with a few pigment cells in it. Blood 
vessels are of frequent occurrence, running in all directions, and 
ranging from very large ones visible to the naked eye to very 
minute capillaries. Even in the open network of connective 
tissue surrounding the hinder part of the cerebellum, and con- 
stituting the cephalic portion of the structure in question, blood 
vessels ramify quite freely. The pia mater on the sides of the 
medulla, and surrounding the cerebellum, is almost a solid mass 
of blood vessels, which are densely crowded with corpuscles, so 
crowded, in fact, that they appear as solid masses of tissue, and 
their identity was for some time in doubt. Caudally, the open 
network of connective tissue is lost entirely, and the gland 
appears solid throughout, and denser than the portion drawn 
(pl. 11, fig. 2). 
