DEVELOPMENT AND LIFE- HISTORIES OF TELEOSTEAN FISHES. 799 



skeletal — is subsequently assumed, and it must be added the paired sensory (nerve) con- 

 nection, above mentioned, is probably also secondary, for in Selachians and Dipnoans no 

 trace of it is seen. It is remarkable that in some forms, e.g. , Apeltes, vascular loops are 

 formed in the median fin-folds at the time of hatching, whereas in none of the species 

 specially referred to in this paper is this the case — the membranes remaining transparent 

 and non-vascular for some time after extrusion. 



The Caudal Fin. — It is plain from the foregoing observations that the tail of the 

 embryo is not by any means distinctly marked off from the trunk. The tail is indeed 

 the tapered portion of the trunk, which gradually diminishes, ending posteriorly as a thin 

 rod (the notochord with the muscle-plates on each side), and the neurochord above (cf. PI. 

 XIII. figs. 1, 2, 4, 6a, 7). A strand of connective tissue passes along beneath the rod, 

 and in this tissue the haemal trunks of the tail are by and by formed (vs., PL VII. fig. 6a ; 

 PL XL figs. 15, 17). The whole is encircled by the embryonic fin-membrane (ef) 

 which passes along the median dorsal, terminal, and ventral line, so that the tail is at 

 this early stage of the typical protocercal type, showing no division into lobes. In the 

 early ling, about the time that the eyes are fairly complete, two peculiar folds are sent off 

 below the muscle-plates in the caudal region. While within the ovum the caudal trunk 

 lies for some time as a flattened process upon the yolk, its greatest breadth being at right 

 angles to the caudal plane of symmetry, and when first it buds out from the trunk it 

 is in a state of torsion, the developing fin-membrane being folded in a complicated manner 

 at the root of the tail, and passing as a horizontal ridge round its termination (PL II. 

 fig. 11). This state of torsion, which is very marked in the earliest condition of the tail, 

 does not continue, and shortly before hatching the enlargement of the perivitelline space 

 not only gives the caudal trunk more freedom, but even permits active movements on 

 the part of the embryo. 



Usually, as pointed out above, the trunk terminates in a more or less accuminate 

 process (PL XIII. figs. 1, 2, 4, 6a, 7); but in PL XV. fig. 4, a remarkable terminal 

 enlargement is seen, the neurochord swelling to form a lobe, while the notochord ends in 

 an irregular bulbous structure. In the figure just referred to (PL XV. fig. 4) the tail- 

 fin proper is marked by a radial structure (embryonic fin-rays), apparently a mere 

 dermal thickening, such as we see in a late stage of Pleuronectes limanda (PL XVI. fig. 

 3). In PL XIII. figs. 6, 6a, the embryonic membrane is diminished between the ter- 

 minal caudal and anterior portions, and a mass of granules is forming around the end of 

 the notochord, which assume a radial disposition. These diverging granular tracts are 

 better defined, and form, in fact, rays in the dorsal and ventral lobes of the membranous 

 fin of the same embryo (PL XIII. fig. 7). The formation of fin-rays, without the 

 intervention of special cellular prolongations from the vertebral arches, was observed by 

 Lereboullet, who speaks of them as produced probably by " the deposit of a cartilaginous 

 cytoblastema " (No. 93, p. 634). The appearance of these rays does not suggest a 

 cartilaginous character, the fine granular tracts (PL XV. fig. 5), as they become defined- 

 form clear translucent rods (PL XIX. figs. 2-4), not unlike the " spicular substance," 



