774 PROFESSOR W. C. M'INTOSH AND MR E. E. PRINCE ON 



Anus. — The anus in the forms here described is not a proctodseum, as it is not 

 produced by the ingrowth of the external epiblast, but is at first a lateral opening (see 

 PL VII. figs. 12-15), which five or six days after hatching is formed by the protrusion 

 of the anal section of the alimentary canal. In Molva vulgaris, early on the second day 

 after emerging, the anal tract seems still to end blindly, being continued backward 

 nearly in a straight line, or in some cases sending down a terminal process at right angles 

 to the main axis of the canal. This terminal prolongation is carried down to the middle 

 of the marginal fin, and generally on the second or third day is found to break through 

 in a manner not unlike the oral opening. The rectum is thus a capacious thick-walled 

 tube, sending out a narrow anal continuation consisting of a fine tube lined by a single 

 layer of cubical epithelium, and it passes through the thick tenacious plasma contained in 

 the space behind the urinary vesicle (PL VII. figs. 12, 13). This space is enclosed between 

 the two epiblastic lamellae of the caudal membrane, and the anal tube curves round and 

 opens laterally on the surface of the latter, some distance from the ventral margin. 

 Later the membrane below the aperture becomes absorbed, the rectum assumes thicker 

 walls {Jig, PL VII. figs. 8, 9), and the usual muscular rectal portion of the alimentary 

 canal is formed during the second week after emerging. The anus then opens in the 

 ventral middle line, as in the adult fish. 



Liver. — Soon after the otocysts are formed the ventral wall of the mesenteron in its 

 fore part shows an enlargement — " an ovoid dilatation just before and below the early 

 pectorals," according to Lereboullet (No. 93, p. 584), and his description holds to a 

 large degree for pelagic Teleosteans. Certainly the liver is a distinct outgrowth from 

 the ventral wall of the mesenteron. Hoffman has expressed the view that the liver 

 originates from the yolk-periblast, and that the hepatic diverticulum is really a prolifera- 

 tion of " parablast entoderm" (No. 69a). Such sections as fig. 2, PL VII., do not 

 support this view, for the periblast (per) is a distinct, granular layer beneath, and 

 separated by a delicate stratum of hypoblast (Jiy) from the cells which build up the liver. 

 The liver, in fact, is largely a solid proliferation of the ventral wall of the mesenteron, and 

 is periblastic, or formed of " parablast entoderm " only in the degree that the ventral wall 

 of this region is periblastic, and this we have seen at this point to be at a minimum. 

 Into the early liver (Ir, PL VII. fig. 5) a delicate canal (dc) passes, a direct prolongation 

 of the enteric lumen, doubtless the ductus choledochus. Lereboullet noticed this 

 especially when the mesenteron dilated and contracted as it does in later embryonic 

 stages (No. 93, p. 593). In Perca, on the sixth day, the same observer describes 

 numerous ramifying fissures or prolongations from this delicate canal; and the gall- 

 bladder he regards also as a tubular outgrowth of the intestine. The hepatic pro- 

 liferation becomes bifid, a dorsal and a sinistral ventral lobe being distinguishable. The 

 liver also becomes divided into small lobuli {J/r, PL VII. figs. 1-3), in the midst of which 

 the spacious gall-bladder (gb) appears as a clear vesicle, limited by an epithelial wall 

 of a single layer of cells. 



Swim-Bladder. — From the dorsal wall of the mesenteron (mg) the swim-bladder (sb) 



