192 EDWARD PHELPS ALLIS, JUN. 



23orfcion of the entire anterior diverticulum of the fish. The 

 aperture represents^ in fact, that middle point of the entire 

 diverticulum of Wright's descriptions where, according to 

 that author, the lumen of the diverticulum becomes suddenly 

 constricted. From here the diverticulum in my embryo led 

 upward and forward until it acquired a position slightly 

 dorso-lateral to the ventro-mesial diverticulum. There it 

 continues forward until it reaches the anterior end of the 

 ventro-mesial diverticulum, where it first takes a sharp turn 

 dorso-laterally, then another forward again, and ends. Its 

 dorso-anterior portion, distal to the first sharp bend, is some- 

 what enlarged, as Wright has said, and is lined with what he 

 describes as a " modified epithelium." The entire diver- 

 ticulum may be called the dorso-mesial diverticulum of the 

 fish. 



The ventral opening of the pocket from which these two 

 mesial diverticula arise lies mesial to the spiracular pseudo- 

 branch. Lateral to that pseudobranch, and from the opposite 

 wall of the cleft, the auditory diverticulum of Wright's 

 descriptions has its origin. The pseudobranch, which is, in 

 position, a mandibular hemibranch of the spiracular cleft, 

 thus lies between the regions where these two diverticula 

 have their origins from the cleft. 



The auditory diverticulum runs upward and mesially, at 

 first immediately posterior to the levator maxilla superioris, 

 and then internal to the dorsal portion of that muscle, and 

 reaches the side wall of the skull. There it turns upward 

 and laterally along the sloping side wall of the skull, and, as 

 Ridewood says, broadens in an antero-posterior direction ; its 

 upper end thus being T-shaped, and here lying external to 

 and below the external semicircular canal of the ear. This 

 auditory diverticulum has, in sections of Mustelus, most 

 decidedly the appearance of the antero-dorsal end of the first 

 branchial (hyobranchial) cleft of sections of larvee of Amia. 

 In this latter fish this antero-dorsal end of the first branchial 

 cleft is a dorsal pocket, or cranial extension, of the cleft, and 

 it almost reaches the side wall of the skull in the otic region 



