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Zoology Publications from Victoria University of Wellington—Nos. 54 to 57

Development of the Egg

Development of the Egg

The mean dimensions of 100 eggs were 1.65 × 1.35 mm. The eggs are oval and dorso-ventrally depressed and are attached to the substrate by a flattened adhesive base. When first laid the eggs are bright crimson and are about 1.2 mm long, but within minutes they expand to a length of 1.65 mm, by uptake of water through the egg membrane. The yolk is central and subspherical and has a mean diameter of 1.3 mm. The yolk of the fertilised egg is a crimson-red colour, and contains between 10 and 100 oil globules, one of which becomes dominant during later development. Development to hatching (Figs. 1–2, nos. 1–10) took 12 days at a water temperature of 15°c.

Two hours (Fig. 1, no. 1). A single large cell appears from beneath the yolk and later expands and moves to one end of the yolk within one of the lateral perivitelline cavities.

Three hours (Fig. 1, no. 2). The first cleavage divides the cell dorso-ventrally relative to the egg base, and at right angles to the yolk surface. The cells formed round off and appear as two swellings protruding from one end of the yolk-sac.

Four hours (Fig. 1, no. 3). The four celled stage is produced by a second cleavage at right angles to the first. Oil globules are free to move and tend to migrate to the uppermost region of the yolk; hence their position depends on the attitude of the egg.

Five hours. In the eight celled stage the blastomeres are still in a single layer. The cells are arranged in two rows of four cells each.

Six hours. The sixteen celled stage is reached with the cells beginning to form a round blastodisc.

Twenty hours (Fig. 1, no. 4). The blastula is well formed and consists of a prominent cap of cells, beneath which lies the blastocoel.

Forty hours (Fig. 1, no. 5). At this stage the blastodisc has spread halfway around the yolk, in so doing obliterating the blastocoel. Epiboly is not obvious, except for the slight thickening of the lateral rim of the blastodisc.

Sixty-six hours (Fig. 1, no. 6). The blastodisc covers the entire yolk-sac. The primitive streak is well defined and lies deeply notched into the yolk, particularly in the cephalic region.

Eighty-five hours (Fig. 2, no. 7). The outline of the embryo is distinct, encircling half the yolk-sac. The myotome rudiments and the otic capsules are just visible. The main oil globule has become larger, apparently at the expense of the smaller oil droplets which have slowly decreased in size. Fore-brain develop ment is quite evident.

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Fourth day. At this time the embryo encircles more than half of the yolk and the tail begins to extend free from the yolk-sac. The oil globule lies directly beneath the tail and remains here for the rest of the development. Optic vesicles contain outlines of the lens, and there is a pronounced enlargement of the hindbrain. Approximately 17 myomeres are present.

Fifth day (Fig. 2, no. 8). The total body length has increased slightly. Nearly all the myomeres are present posterior to the otic capsules and extending well down into the tail. At this time the heart begins to beat very faintly, but blood movement is seen only in the region of the heart. The heart lies well forward beneath the head and is obscured by the yolk.

Seventh day (Fig. 2, no. 9). The yolk is reduced considerably and the ventral aspect of the embryo faces upwards. The tail is well formed and is turned back to lie parallel with the body. The gut is formed and has scattered pigment spots on its upper surface. The heart beats regularly and strongly. Each beat of the heart sends a wave of movement through the yolk. Blood flows in the dorsal and ventral blood vessels, and the vitelline vessels are large and run laterally across the yolk. Pigment has appeared in the chorioid of the eye.

Twelfth day (Fig. 2, no. 10). The mouth and external nares are formed and the gut pigmentation has become darker. There is a further reduction in the amount of yolk. Pigmentation of the chorioid appears complete. The embryo is cramped within the egg and the tail arches forward to overlap the head. Just prior to hatching the embryo becomes agitated and begins to flex its tail. As a result of this activity the chorion is ruptured and the prolarva is released.

Prolarvae (Fig. 3, no. 11–12). Some of the larvae that hatched in the laboratory may have been induced to do so prematurely, as a result of disturbance. This is suggested by the variation in the amount of yolk present in each prolarva immediately after hatching. Prolarval length on hatching ranges from 4.8–5.5 mm. At this stage the gut is long and extends to the base of the 15th myomere. The upper peritoneum of the gut is covered with numerous stellate melanophores, extending from above the yolk to the vent. Posterior to the gut are 16–18 myomeres. Stellate melanophores are present in the myomeres just past the vent, but their number is variable. An obvious yellow tinge surrounds the braincase, and this extends through the myotomes above the gut to the 8th myomere past the vent. All larvae kept in the laboratory died within three days of hatching.

Larvae (Fig. 3, no. 13–14). At 6.5 mm the yolk-sac is almost absorbed and the gut reaches beyond the mid-length of the body. The oil globule is no longer visible. Pigmentation has changed very little except for an increase in the size of the melanophore at the base of the pectoral fin. The jaws are well formed and appear functional, although as yet are without well defined teeth. Two gills and 6–7 branchiostegal rays are visible on either side of the head. Two sucker buds lie ventral to the gills and heart. The longitudinal fin folds have slightly increased in size.

At 7.85 mm (Fig. 3, no. 15–16), the overall shape of the larva changes slightly. Essentially there is a flattening and broadening of the head, and an increase in the depth of the tail. In addition to the existing pigment pattern, numerous grey spots are scattered about the outer edges of the larvae. The dorsal, anal and caudal fins show signs of ray formation. The urostyle is curved upwards and extends a considerable distance into the caudal fin. The ventral sucker is well developed and functional. At this small size the larvae are able to cling to the page 5sides of glass jars. No specimens larger than 7.85 mm were caught in the plankton nets. Because larvae of this size have a fully functional sucker (Fig. 4, no. 18) it is suggested that at this stage they attach beneath stones.

The average measurements of twenty-five prolarvae are as follows:
Standard length 5.4 mm
Total length 5.7 mm
Head length 0.95 mm
Eye length 0.41 mm
Snout to anus 3.0 mm
Larval measurements are as follows:
Standard length 6.5 mm 7.85 mm
Predorsal length 3.2 mm> 5.0 mm
Head length 1.4 mm 2.15 mm
Depth of head 0.9 mm 1.21 mm
Snout length 0.31 mm 0.5 mm
Eye length 0.5 mm 0.57 mm
Interorbital distance 0.35 mm 0.55 mm
Head width 1.1 mm 1.4 mm