THE PINEAL BODY 197 



the paraphyseal arch, which extends from the dorsal extremity 

 of the lamina terminalis to the most cephalic depression in the 

 roof, namely, the velum transversum. The portion of the roof 

 immediately caudad of the velum forms the middle or postvelar 

 arch, which in turn is separated from the third or caudalmost 

 arch by a slight depression containing the superior or habenular 

 commissure. This is the epiphyseal arch. In some species a 

 small intercalated portion of modified gray matter inserts itself 

 between the caudal limit of the postvelar arch and the superior 

 commissure. This is the pars intercalaris anterior. Caudally, 

 the epiphyseal arch extends toward the cephalic extremity of 

 the posterior commissure, but between the latter and the caudal 

 extremity of the arch there is interposed a small area of modified 

 gray matter, the pars intercalaris posterior. The caudalmost 

 element in the pineal region is the posterior commissure, and to 

 this, perhaps, should be added the subcommissural organ, recently 

 described by Dendy and Nicolls 88 and others. 



These structures of the pineal region or their homologues exist 

 in all vertebrates either in the embryonic or adult condition. 

 The paraphyseal or prevelar arch is common to all vertebrates. 

 From its caudal portion, i.e., the region of the arch nearest the 

 velum transversum, there develops a specialized structure, the 

 paraphysis. This structure, either in anlage or as an adult 

 organ, appears in all vertebrates. 



In cyclostomes (Kupffer 224 in Ammoccetes, Burckhardt 47 in 

 Petromyzon) the paraphysis is a small sac-like diverticulum, if 

 not itself highly vascular yet in close relation with the vascular 

 mesenchyme immediately above it. In selachians (Minot 277 

 and Locy 243 in Acanthias) the structure is a small outgrowth 

 from the paraphyseal arch. In ganoids (Kupffer 223 in Acipenser, 

 Hill, 180 Eycleshymer and Davis 113 in Amid) the paraphysis is a 

 large diverticulated and vascular organ. In many teleosts 

 (Burckhardt, 47 Studnicka, 391 and Terry) 392 the paraphysis appears 

 to be rudimentary. In dipnoians (Burckhardt 44 in Protopterus) 

 the organ is a wide outgrowth with many small diverticula and 

 rich in blood vessels. The paraphysis in amphibians attains its 

 greatest conspicuity as an organ. It is highly differentiated in 



