DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 77 
Or 
is given off as a very thick-walled diverticulum (PI. VII. fig. 5), which presses upwards 
against the notochord, and remains connected for some time by a fine canal (Pl. VII. 
figs. 2, 4). Before the embryonic period ends, however, the duct atrophies, all the forms 
specially referred to being physoclistous. 
Heart and Circulation.—The heart is developed at a very early stage—before the 
cesophagus is formed—as a cylindrical structure (hr, Pl. IV. figs. 8, 12), in front of the 
pectoral region, 2.¢e., between the otocysts and the optic vesicles. Soon after the alimentary 
tract is defined, or, as WENCKEBACH expresses it (in the case of Belone), after the ventral 
closure of the gut, and when about twenty-four proto-vertebre are marked off, the heart has 
a vermiform shape, and is still solid. This solid condition LerEBouLEr described in Perca, 
but in Sa/mo and other forms the heart is stated to appear in the form of a single or 
double tube (vide Horrmay, No. 69a; Batrour, No. 11, p. 637). That the heart 
develops as a single tube in the Gadoid and other forms here considered is not surprising. 
When the heart arises as two tubes it appears to be connected, as BaLrour pointed out 
(No. 15, vol. xi. p. 689), with the non-closure of the pharynx inferiorly, but in those 
Teleosteans where the cesophageal cavity is formed later by a forward growth of the enteric 
lumen, the solid tract is really closed below, and this is the condition correlated with an 
unpaired cardiac rudiment. Its first indication in the living embryo is seen as a rounded 
projection beneath the solid cesophagus bulging out towards the subjacent periblast. 
It is a ventral outgrowth of that splanchnic mesoblast, which also forms the branchial 
arches. LeEREBOULLET describes this cardiac swelling (on the seventh day in Perca) as 
having its inferior portion, the auricle, resting directly on the yolk (No. 93, p. 584; vide 
his pl. iii. fig. 13). It is a median unpaired projection, and carries down before it a 
very thin layer of hypoblast. At times this delicate stratum of hypoblast cannot be 
made out, and in P. platessa it would appear to be absent; nor can a layer of hypoblast 
be distinguished over the rest of the surface of the yolk, though such a layer is readily 
seen in other Pleuronectids, as well as in Gadoids (Pl. VIII. fig. 11). In all cases, however, 
the continuity of the rudimentary heart and the “ branchial” mesoblast above is 
maintained.* Horrman describes in Salmo two lateral folds of splanchnic mesoblast, 
which pass down beneath the pharynx, and produce by a dorsal and a ventral union a 
tubular heart (No. 69a; vide fig. 9, Taf. ii.). Before the tube is complete inferiorly, some 
intruding cells of “ parablastic entoderm,” i.e., periblast, form the cardiac endothelium 
(No. 69a, Taf. iii. fig. 6; Taf. iv. fig. 6). Such a process does not accord with the appear- 
ance of the heart in the living condition, for in the embryonic cod, haddock, and others 
no lumen is visible at first, as OELLACHER and Gorve also hold, and indeed after the lumen 
is formed the endothelial lining is absent (vide surface-views, Pl. VIII. figs. 3, 7; and 

* WENCKEBACH (op. cit., and Jour. Roy. Mier. Soc., February 1887) describes its first appearance as a band of meso- 
dermic cells close behind the optic vesicle on the lower surface. They arise from the indifferent mesodermic cells of the 
head which wander round the gut. The mass of cells splits to form a kind of pouch—the heart. The blood-vessels 
have a similar mesodermic origin. The heart opens into the segmentation-cavity, and its lumen is nothing else than 
part of the blastoce:l. The blood is mesodermic in origin, he avers, neither endoderm nor free periblast, i.2., nuclei, 
having any share in its formation. 
