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



(PI. IV. figs. 11, 19), they increase until the enteric walls are thick, and include many 

 liters of wedge-shaped cells (PL VII. figs. 7, 9). In P. fiesus of the ninth day {i.e., two 

 days before hatching) the walls are just "00 1 inch in thickness, and the lumen in the 

 middle or widest part measures in horizontal breadth slightly less. A delicate inner layer 

 lines the lumen, which has a granular or mucoid appearance, but it subsequently forms a 

 ciliated enteric lining. It is not more than '000125 inch in thickness. The lumen of 

 the mid gut is large and round in transverse section (mg, PI. VII. fig. 3), but much more 

 depressed further forward. A section through the pectoral region, where the enteron is 

 oval and the lumen a wide transverse fissure, shows a diminished dorso-ventral capacity, 

 while in the oral region proper a mere horizontal slit extends from side to side of the 

 wide and very much depressed layer of oesophageal hypoblast (PI. XL figs. 2-8). The 

 tract is thus a closed sac (PL IV. fig. 12), flattened anteriorly, round and cylindrical poste- 

 riorly, the mouth and anus being " the last parts," as Lereboullet said, "to be formed." 

 Around this tube of hypoblastic cells the splanchnopleure (sp) grows, forming a thin 

 external sheet which pushes in below the notochord, and cuts off that structure from the 

 mesenteron (PL VII. fig. 6). These mesoblastic cells do not become a fibrous layer for 

 some time, but later they give origin to the muscles of the canal and its connective-tissue, 

 while externally they give rise to the epithelial peritoneal layer. 



In the oesophageal region the course of the hypoblastic cells is extremely difficult to 

 follow. They give origin to a cardiac swelling which is sub-oral and median (hr, PL IV. 

 fig. 13; PL V. fig. 8), while other cells pass into the hypoblast laterally to form the 

 core of the visceral folds. During the first few days after hatching the anus is still 

 undifferentiated, as Lereboullet found to be the case in Perca ; nor is the oral cavity 

 externally open, as the same observer also proved in Perca by experiments with various 

 colouring matters (e.g., indigo), the alimentary tract being in fact a closed cylinder, con- 

 sisting of a very thick inner wall of cylindrical cells (hy, PL IV. fig. 11), whose free 

 rounded ends project into the cavity of the gut (fg), and externally of a thin layer of 

 flattened mesoblastic cells (sp) not yet transformed into muscular and other tissues (vide 

 No. 93, p. 625). 



Many preparations show a lining apparently of cilia,* and there is thus great probability 

 that the enteric tract — the oesophageal portion at least — of young Teleosteans is ciliated. 

 Its walls for some time are straight and smooth, but in later stages folds and wrinkles are 

 formed, the intestine especially showing a complexly folded internal surface (PL XIV. fig. 5 ; 

 PL XVIII. figs. 1, 11). The various parts of the tract become rudely marked during the 

 first week after hatching. Thus a gurnard on the thirteenth day (PL VII. fig. 9) shows 

 very distinctly a capacious though depressed oral chamber, the floor of which is ridged by 

 the branchial bars and hyoidean framework, followed by a wide oesophagus (fg), the lumen 

 of which is so flattened as to be little more than a horizontal fissure in transverse section. 

 From this portion the duct of the swim-bladder passes (PL VII. fig. 4).t The enlarged 



* Shipley has recently described the oesophagus in Petromyzon (47th day) as ciliated (No. 150, p. 351). 

 t Vide the highly suggestive remarks of Prof. Cleland on Teleostean pneumatic ducts — Memoirs, &c, in 

 Anatomy, 1889, p. 170. 



