30 FREDERICK TILNEY AND LUTHER F. WARREN 



anterior is observed, but there is a well-marked commissura 

 habenularis as well as the pineal organ, a posterior intercalated 

 portion, and the posterior commissure. 



The earliest work upon this region of the dipnoi was by 

 Huxley 191 in 1876. In this he described the pineal organ as a 

 cylindrical structure which had a cordiform enlargement at its 

 distal extremity. This latter lay deeply seated in a small exca- 

 vation of the cartilaginous skull roof. Wilder 427 in 1887 showed 

 an unusually large paraphysis in Ceratodus, but did not observe 

 the pineal organ. Sanders 343 in 1889 saw the end^ vesicle of the 

 pineal organ in the form of a small body situated above the 

 chorioid plexus of the interbrain. Studnicka ('95, '96), 386 

 distinguished in Ceratodus a dorsal sac and a paraphysis, the 

 former lying closely compressed against the latter. He also 

 observed a pineal organ with a long stalk which lies in a fold 

 along the superior wall of the dorsal sac, while the end-vesicle 

 is situated above the paraphysis. In Protopterus annectens, 

 Wiedersheim ('80) 423 and Beauregard ('81) 19 mistook the dorsal 

 sac for the pineal organ, and Fulliquette ('86) 132 was unable to 

 distinguish between the ganglion habenulae and the pineal 

 organ. The erroneous identifications made by these authors go 

 to show the great difficulties which the pineal region in dipnoians 

 presents. It was not until 1890 and 1892 that Burckhardt 42 " 44 

 first gave the proper description of the pineal organ in these 

 forms. 



6. The pineal region in amphibia 



In Urodela and Apoda only the pineal organ develops and 

 this in but an extremely rudimentary form. The portions of the 

 pineal organ which are present in these forms represent the 

 proximal part of that structure. In no other group of verte- 

 brates is the pineal organ so little developed; it presents itself as 

 a sac lying close to the interbrain, the lumen of which is sub- 

 divided into numerous branches. deGraaf 155 in 1886 was first 

 to recognize this condition and describe it in amphibia. 



In Anura, as in Urodela and Apoda, the pineal organ only 

 develops. It usually consists of the proximal saccular part of 



