1911] Chandler: A Lymplioid 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 like 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 spec- 

 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 slightly smaller in proportion to the 

 brain than in older individuals. 



* 



Sl'.MMARY 



1. A lymphoid, "gland-like 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^jnyelencephalic gland (provisionally so named) is 

 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. 



