DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 775 



is given off as a very thick-walled diverticulum (PL 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 

 oesophagus is formed — as a cylindrical structure (hr, PL IV. figs. 8, 12), in front of the 

 pectoral region, i.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-vertebrse are marked off, the heart has 

 a vermiform shape, and is still solid. This solid condition Lereboullet described in Perca, 

 but in Salmo and other forms the heart is stated to appear in the form of a single or 

 double tube (vide Hoffman, No. 69a,; Balfour, 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 Balfour pointed out 

 (No. 15, vol. xi. p. 689), with the non-closure of the pharynx inferiorly, but in those 

 Teleosteans where the oesophageal 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 oesophagus bulging out towards the subjacent periblast. 

 It is a ventral outgrowth of that splanchnic mesoblast, winch also forms the branchial 

 arches. Lereboullet 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 pi. 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.* Hoffman 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 Gotte 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. Micr. 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 rnesodermie 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 rnesodermie origin. The heart opens into the segmentation-cavity, and its lumen is nothing else than 

 part of the blastoccel. The blood is rnesodermie in origin, he avers, neither endoderm nor free periblast, i.e., nuclei, 

 having any share in its formation. 



