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 recogniz- 

 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, pi. 2, fig. 7), Parker and 

 Balfour (1882, pi. 25, fig. 47u), and Allen (1907, pi. 6, 'figs. 11 

 and 12). Kingsbury (1897, pi. 6, fig. 5) has a diagrammatic 

 sketch 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 

 orijan 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, Scaphirkynchus, Polyodon, and Amia. 

 In none of these, however, could any indication of it be found, 



