DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. tis 
At the sides of this space, 7.e., beneath the eyes, the hypoblast becomes thickened 
as two lateral longitudinal ridges (PI. IX. fig. 1; Pl. XI. 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. 5b, 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 
hypoblastie 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 cesophageal 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 Horrmay, 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 cesophageal 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. Lrresou.er 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 végétation celluleuse,” such as Voor had described as involved in the 
formation, not only of the intestinal tube, but of the liver and kidneys (No. 93, p. 538). 
Dourn 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 optie 
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, LEREBOULLET'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 (PI. IV. fig. 11). The cells now assume the full cubical 
columnar character characteristic of the enteric epithelium, and, at first a single layer 
