90 University of California Publications in Zoology 



In Lepisosteus platystomus the gland is almost identical in 

 form to that in L. osseus, but the ear-like projections are rela- 

 tively shorter and stouter than in any specimen of the latter 

 species which I have examined. 



The size of the gland as compared with the rest of the brain 

 is considerable. The width of the main body in a 25 cm. speci- 

 men is slightly greater than the width of the optic lobes, which 

 form the widest dimensions of the brain, while the width from 

 tip to tip of the lateral lobes is more than twice that of the 

 cerebrum at its greatest width. Its length is approximately that 

 of the cerebellum, and its thickness such that it attains the level 

 of the dorsum of the cerebellum. In adults the size of the gland 

 as compared with the rest of the brain is even greater than this. 



The entire brain of Lepisosteus is covered by a deeply pig- 

 mented membrane which has a peculiar metallic appearance, in 

 which the enormous pigment cells appear under the binocular 

 microscope as a sprinkling of pepper. This covering membrane 

 seems to be directly continuous with the covering of the myelence- 

 phalic gland, or with the gland itself, since in microscopic section 

 there is no distinctly differentiated outer covering. In other 

 words, the structure seems to be a very highly developed and 

 enlarged portion of the pigmented covering of the brain. 



HISTOLOGY 



In order to determine the histological structure of the mye- 

 lencephalic gland, the best specimen available was fixed in for- 

 malin, imbedded in paraffin, and cut in sections 12 microns in 

 thickness. The sections were then stained in Delafield's haemo- 

 toxylin, and counter-stained with eosin or with picro-fuchsin. 



When placed under the microscope, it became at once ap- 

 parent that the structure under study was not of nervous tissue, 

 but appeared as an enormous development of the pial covering 

 of the brain. In some of the sections there appeared to be a 

 cavity, but this proved to be nothing more than a very large 

 central blood vessel or sinus. In the section drawn (pi. 10, 

 fig. 1) the true pia mater is indistinctly differentiated from the 

 gland across the dorsum of the fourth ventricle and is fused 

 with it. Where the pia mater curves down over the side of the 



