DEVELOPMENT AND LIFE-HISTOKIES OF TELEOSTEAN FISHES. 771 



At the sides of this space, i.e., beneath the eyes, the hypoblast becomes thickened 

 as two lateral longitudinal ridges (PL IX. fig. 1 ; PL XL fig. 1), but elsewhere in this 

 region the layer forms a very thin plate (PL IV. fig. 21). That the roof of the primi- 

 tive enteron is thus formed as a dorsal sheet of invaginated hypoblast, admits of no 

 doubt. Such sections as figs. 56, 6, and 10, PL IV., demonstrate this, and the ventral 

 wall of the canal is formed of cells either pushed in from the side — that is, formed of true 

 hypoblastic cells or aggregated as masses of protoplasm around the scattered nuclei of the 

 periblast, and budded off. While the posterior portion is formed in this way, the mesen- 

 teron proper appears to develop in a different manner, being formed by a multiplication 

 of the invaginated (hypoblastic) cells ; and a ventral and a dorsal wall are not definitely 

 formed from periblastic and hypoblastic cells respectively, but doubtless periblastic cells 

 contribute in some degree to build up this portion of the tract also, though in such 

 sections as figs. 13 and 14, PL IV., the hypoblast (hy) is a very definite and continuous 

 layer. The mid and fore portions form a dense cord, in which a lumen appears later by 

 the forward extension of the posterior enteric chamber, this oesophageal slit extending in the 

 ling, two days old, in front of the otocystic region. At first the hind gut is open to the 

 yolk below (as in PL IV. figs. 5b, 6), but no sections show this to be true of the enteric 

 tract further forward. According to Hoffman, paired involutions of hypoblast produce 

 the tract which he thus holds to be open to the yolk beneath (vide No. 69a, Taf. i. fig. 3), 

 but no section in pelagic forms indicates such a mode of origin of the mesenteron, and 

 certainly not of its oesophageal portion. The earliest condition of the alimentary tract is 

 a continuous sheet of hypoblast, thickened on each side in the oral region to form the 

 lateral walls of the oral chamber. Lereboullet regards the alimentary canal as developed 

 by a folding-in of the " mucous layer," though the pharyngeal section, he holds, is not 

 formed till later. In his earlier researches he states that the enteric tract is possibly 

 formed by " une vegetation celluleuse," such as Vogt had described as involved in the 

 formation, not only of the intestinal tube, but of the liver and kidneys (No. 93, p. 538). 

 Dohrn believes that the oral hypoblast is a forward growth of the mesenteric mass, nor 

 is there evidence to show that this is not so. At any rate, in the embryo whose optic 

 vesicles are in process of formation, the hypoblast (hy, PL IV. figs. 4, 13, 14, 20) is a thin 

 sheet — a single layer of cells for the most part over the entire ventral surface, save at the 

 posterior extremity (hy, PL IV. fig. 10). At a somewhat later date, when the invagina- 

 tion of the lenses is completed, the mesenteron is a massive cylinder, and the oral tract a 

 wide flattened sheet of hypoblast formed either by proliferation of the invaginated layer, 

 or by forward growth of the denser hind gut, probably a combination of both. In any 

 case, Lerebotjllet's view is the correct one, viz., that the pharynx is a separate and later 

 formation than the mesenteron proper. By the time the walls of the otocysts have 

 thinned out and their chamber has enlarged and contains the otoliths, a fine horizontal 

 fissure traverses the pharynx, and a lumen thus continues from the oral to the blind anal 

 end of the alimentary canal (PL IV. fig. 11). The cells now assume the full cubical 

 columnar character characteristic of the enteric epithelium, and, at first a single layer 



