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Zoology Publications from Victoria University of Wellington—Nos. 63 and 64

Development of the Egg and Prolarva

Development of the Egg and Prolarva

The egg is essentially spherical although slightly dorsoventrally depressed. The flattened base is covered with numerous fine, translucent, adhesive threads. The mean chorion diameter is 1.07mm (100 eggs measured) and ranges from 1.00mm - 1.10mm. Early in development the yolk is spherical and averages 0.90mm and contains 80-120 small oil globules closely associated with 20-25 pigment spheres. No cluster was found with eggs earlier than the blastula stage and these were estimated to be about thirty hours old. Development to hatching (Fig. 1, Nos. 1-9) took 17 days at a temperature ranging from 11°C-13.5°C.

Thirty hours (Fig. 1, No. 1). The blastula is well formed and consists of a rounded cap of cells (blastoderm) overlying a deep blastocoel. Pigment spheres and associated oil droplets are distributed evenly throughout the yolk.

Second day (Fig. 1, No. 2). The blastodisc has spread half way around the yolk. The embryonic shield is poorly defined, appearing only as a thickened portion of the blastodisc.

Third day (Fig. 1, No. 3). The blastopore has closed. The optic vesicles are present but rudimentary. The embryo is deeply notched into the yolk, particularly in the head region, and the tail bud is flat. Several of the yolk pigment spheres have coalesced, as have some of the oil globules, effectively reducing slightly the number of each.

Fifth day (Fig. 1, No. 4). The pericardial cavity is well defined and the heart beats faintly, but there is no visible blood circulation. The brain is lobed and the ventricles pronounced. A cluster of small stellate melanophores lies anterior and posterior to each eye; the greatest number lie posterior to the eyes. The chorioid fissure is not closed. The auditory placodes appear as two small rings lateral to the hind brain. Behind each placode there is a group of about 10 small melanophores. Two more pigment clusters lie half way along the embryo, and several scattered melanophores are present in the tail bud. Pigment spheres and oil globules continue to coalesce.

Sixth day (Fig. 1, No. 5). The heart beats strongly and rapid circulation is visible in the dorsal and ventral blood vessels and the vitelline tributaries. Brain lobes have expanded laterally, especially the mesencephalon, and two otoliths are present within the otic vesicles. The tail extends well clear of the yolk and the gut is long and tubular. Several stellate melanophores surround the gut and extend in two rows along the ventral aspect of the tail. These melanophores are obscure and cannot be accurately counted. Ten to 15 stellate melanophores lie scattered over the yolk surface. Twenty to 30 oil droplets and 3-4 pigment spheres remain within the yolk. The relative position of the oil globules in the yolk varies according to the attitude of the egg because the oil floats to the uppermost portion of the yolk.

Eighth day (Fig. 1, No. 6). The heart now lies slightly forward of the head. The chorioid fissure has closed and eye pigmentation is just visible. The general arrangement of the body pigment has altered very little. A large pigment patch is present at the posterior end of the gut, and a single row of 5 stellate melanophores runs along the ventral midline of the tail. Beginning directly above the anus 6 stellate melanophores run along the dorsal mid-line of the tail. Several melanophores are visible above the hind portion of the mesencephalon. All pigment spheres have disappeared from within the yolk. The tubular gut is longer.

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Tenth day (Fig. 1, No. 7). The mesencephalon has expanded dorsally. The chorioid pigmentation is extensive although the lenses are still visible through it. The number of scattered melanophores present anterior and posterior to the eyes, above the gut and laterally within the tail is reduced. Two are present above the brain and 4 behind each otic vesicle. The melanophores along the dorsal mid-line are reduced to 3 or 4 while those on the ventral mid-line have increased to 11 or 12. Rudimentary pectoral buds and fin folds are visible.

Twelfth day (Fig. 1, No. 8). A further reduction in the number of melanophores above the gut, over the yolk surface, and anterior and posterior to the eyes has taken place. Those melanophores situated around the eyes adhere closely to the back of the eye and are somewhat obscured by the chorioid pigment. One or two melanophores lie along the lateral aspect of the body directly behind each pectoral bud.

Fourteenth day (Fig. 1, No. 9). The yolk is considerably reduced, more dense and contains a single large oil globule and 13-15 smaller ones. The head is raised from the yolk revealing the chambers of the heart and the lower jaw. Pigmentation of the chorioid appears complete with many iridiophores present. The amount of body pigmentation varies. Many individuals have only those melanophores found above the gut and along the mid-ventral and mid-dorsal lines. Other specimens still possess the clusters behind each otic capsule and the 8 melanophores around the base of the brain and between the eyes. The mouth, external nares, pectoral fins and fin folds are well formed. By the 16th day (hatching) there is no pigment beneath the brain or behind the otic capsules. Just before hatching the embryo turns violently within the egg at frequent intervals. Eventually the tail penetrates the chorion, freeing the new prolarva.

Prolarva (Fig. 1, Nos. 10 and 11). Prolarval length on hatching ranges from 4.50mm - 5.25mm s.l. The amount of yolk present varies, being influenced by the degree of premature hatching caused by disturbance of the egg, such as by changing the water. The gut is long and convoluted. Above the gut, within the peritoneal layer, are two large melanophores, one directly anterior to the vent and the other above the hind portion of the liver. Some specimens also possess a melanophore beneath the gut and slightly anterior to the vent. The reduced yolk is bordered anteriorly by the heart and posteriorly by the liver and prominent green gallbladder. Sixteen to 26 (most commonly 21) stellate melanophores are present along the ventral mid-line behind the anus. Two to 5 dorsal melanophores are present in the tail. Two stellate pigment spots are situated either side of the anterior part of the yolk. An occasional specimen was found in which the melanophores around the brain and eyes and behind the otic capsules were still present, but these melanophores rapidly faded.