1911] Chandler: A Lymphoid Structure in Lepisosteus 91 



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 gland. 



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 (pi. 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 retic- 

 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 

 (pi. 11, fig. 2). 



