A Guide to the Study of Fishes, Volume 1 (of 2)

by David Starr Jordan

Fig. 387.—Lower jaw of Neoceratodus forsteri Günther. (After Günther.)

Development of Neoceratodus.—From Dean's "Fishes, Recent and Fossil," pp. 218-221, we condense the following account (after the observations of Dr. F. Semon) of the larval history of the Barramunda, Neoceratodus forsteri:

It offers characters of exceptional interest, uniting features of Ganoids with those of Cyclostomes and Amphibians.

The newly hatched Neoceratodus does not strikingly resemble the early larva of shark. No yolk-sac occurs, and the distribution of the yolk material in the ventral and especially the hinder ventral region is suggestive rather of lamprey or amphibian; it is, in fact, as though the quantum of yolk material had been so reduced that the body form had not been constricted off from it. The caudal tip in this stage appears, however, to resemble that of the shark, and, as far as can be inferred from surface views, a neurenteric canal persists. Like the shark there then exists no unpaired fin; the gill-slits (five?) are well separated and there is an abrupt cephalic flexure. In this stage pronephros (primitive kidney) and primitive segments are well marked, and are outwardly similar to those structures in Ganoid; the mouth is on the point of forming its connection with the digestive cavity; the anus is the persistent blastophore; the heart, well established, takes a position, as in Cyclostomes, immediately in front of the yolk material.

In a later stage the unpaired fin has become perfectly established, the tail increasing in length; the gill-slits have now been almost entirely concealed by a surrounding dermal outgrowth, the embryonic operculum; a trace of the pectoral fin appears; the lateral line is seen proceeding down the side of the body; near the anal region the intestine[165] becomes narrower, and the beginnings of the spiral valve appear. In a larva of two weeks a number of developmental advances are noticed; the fish has become opaque; the primitive segments are no longer seen; the size of the yolk mass is reduced; the anal fin-fold appears; sensory canals are prominent in the head region; lateral line is completely established; the rectum becomes narrowed; and the cycloidal body-scales are already outlined. Gill-filaments may still be seen beyond the rim of the outgrowing operculum. In the ventral view of a somewhat later larva the following structures are to be noted: the pectoral fins, which have now suddenly budded out,[166] reminding one in their late appearance of the mode of origin of the anterior extremity of urodele; the greatly enlarged size of the opercular flap; external gills, still prominent; the internal nares, becoming constricted off into the mouth-cavity by the dermal fold of the anterior lip (as in some sharks); and finally (as in Protopterus and some batrachian larvæ) the one-sided position of the anus.

The larva of six weeks suggests the outline of the mature fish; head and sides show the various openings of the tubules of the insunken sensory canals; and the archipterygium of the pectoral fin is well defined. The oldest larva figured is ten weeks old; its operculum and pectoral fin show an increased size; the tubular mucous openings, becoming finely subdivided, are no longer noticeable; and although the basal supports of the remaining fins are coming to be established, there is as yet little more than a trace of the ventrals.

The early development of a lung-fish has thus far been described (Semon) only from the outward appearance of the embryo. The egg of Neoceratodus has its upper pole distinguished by its fine covering of pigment. From the first fine planes of cleavage it will be seen that the yolk material of the lower pole is not sufficient to prevent the egg's total segmentation. The first plane of cleavage is a vertical one, passing down the side of the egg as a shallow surface furrow, not appearing to entirely separate the substance of the blastomeres, although traversing completely the lower hemisphere. A second vertical furrow at right angles to the first is seen from the upper pole. The third cleavage is again a vertical one (as in all other fishes, but unlike Petromyzon), approximately meridional; its furrows appear less clearly marked than those of earlier cleavages, and seem somewhat irregular in occurrence. The fourth cleavage is horizontal above the plane of the equator. Judging from Semon's figure, at this stage the furrows of the lower pole seem to have become fainter, if not entirely lost. In a blastula showing complete segmentation the blastomeres of the upper hemisphere are the more finely subdivided. In the earlier stage the dorsal lip of the blastopore is crescent-like; in the later the blastopore acquires its oblong outline, through which the yolk material is apparent; its conditions may later be compared to those of a Ganoid.

The next change of the embryo is strikingly amphibian-like; the medullary folds rise above the egg's surface, and, arching over, fuse their edges in the median dorsal line. The medullary folds are seen closely apposed in the median line; hindward, however, they are still separate, and through this opening the blastopore may yet be seen. At this stage primitive segments are shown; in the brain region the medullary folds are still slightly separated.