MUSTELUS L^VIS. 191 



Spiracular Cleft. 



The spiracular cleft of a 5 cm. embryo of Mustelus, with 

 its two diverticula, has been described by Wright (70). 

 RideAvood (54) has since made short reference to these diver- 

 ticula in the adult Mustelus, but, although referring to 

 several earlier authors, he apparently was not acquainted 

 with Wright's paper. In my 12' 2 cm. embryo I find both of 

 these diverticula, and also what seems to be a third one ; and 

 the structures deserve a fuller description than Wright has 

 given. 



What Wright describes as the ventral portion of the 

 anterior diverticulum is, in my embryo, a relatively large 

 pocket projecting dorso-mesially from the antero-dorsal sur- 

 face of the spiracular cleft, near its inner end. From the 

 dorso-antero-mesial portion of this pocket two small apertures, 

 lying one directly posterior to the other, lead, the one forward 

 and the other forward and upward, into wholly separate diver- 

 ticula. 



The anterior aperture leads into a small diverticulum which 

 runs almost directly forward through sixteen sections of my 

 embryo. It is at first of oval section, but before it ends it 

 becomes a simple sHt, which inclines mesially and upward 

 toward the skull. The diverticulum has at its extreme 

 anterior end two short but separate and distinct prolonga- 

 tions, or lobes, which extend through two sections only. The 

 diverticulum may be called, because of its position, the ventro- 

 mesial diverticulum of the fish. It is not described by Wright. 

 It is lined with an epithelium which, near the posterior end of 

 the diverticulum, seems to resemble exactly that of the spira- 

 cular cleft. Toward the anterior end of the diverticulum the 

 lining membrane has somewhat the character of the " modified 

 epithelium " that Wright describes in his so-called anterior 

 diverticulum. 



The posterior aperture of the spiracular pocket of my 

 embryo leads into what Wright has described as the distal 



