772 PROFESSOR W. C. M‘INTOSH AND MR E. E. PRINCE ON 
(PL. IV. figs. 11, 19), they increase until the enteric walls are thick, and include many 
layers of wedge-shaped cells (PI. VII. figs. 7, 9). In P. flesus of the ninth day (z.e., two 
days before hatching) the walls are just ‘001 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, Pl. 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 cesophageal hypoblast (Pl. 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 LereBouLLEr 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 (PI. 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 cesophageal 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 (fy), 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 cesophageal 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 cesophagus (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).+ The enlarged 
* Suiprey has recently described the cesophagus in Petromyzon (47th day) as ciliated (No. 150, p. 351). 
+ Vide the highly suggestive remarks of Prof. CLeLanp on Teleostean pneumatic ducts—Memoirs, dc., in 
Anatomy, 1889, p. 170. 
