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Part III. — Ninth Annual Report 
which the brain has undergone from the time when the first divisions are 
well marked (fig. 10) to the larval condition at fig. 21. 
The first of the sense-organs to be preformed is the eye {op.)^ which, early 
on the fifth day (fig. 6), is indicated as an optic lobe. The optic lobes are 
hollowed out as optic vesicles towards the end of the sixth day (fig. 9), and 
twelve hours afterwards (fig. 10) the invagination of the eye has taken place, 
and the lenses have been formed. At the period of hatching (fig. 15) the 
iris (?r.) is beautifully iridescent, and envelops the lens (Z.) except at the 
choroid fissure {cli.). In fig. 16 the head is shown slightly irregular in 
shape, the twisting and unequal development exhibited by the head of 
the Pleuronectidse being anticipated by a considerable inverval. This 
early twisting is abnormal. In the later larval stages the lens becomes 
enlarged, and sometimes appears as if it projected more from the eye. 
The auditory organ {au.) does not appear till two or three days after the 
indication of the formation of the optic organs (fig. 9), and at first is 
situated relatively far back ; but as development progresses and curvature 
of the brain increases, it comes to lie close behind the brain-mass (fig. 
21). Primarily a solid mass which becomes hollowed out to form a simple 
vesicle (fig. 9), strands of tissue appear in theauditory vesicle (figs. 17-21), 
which are tri-radiate in shape. The olfactory organ {ol.) first appears (fig. 9) 
as a solid mass of tissue in the angle between the optic vesicle and fore 
brain, and the cells (fig. 10) are soon arranged radiating from a centre on the 
epidermis. This is due to the olfactory organ becoming an open pit, and 
as such persists in the later embryological stages. 
The heart {lit.) and the digestive system {g.) are well marked at the period 
of hatching, the former appearing as a tube on the ventral surface behind 
the eye and in front of the yolk-sac. By the time of the absorption of 
the yolk the distinction of the circulatory organ (fig. 21) into posterior 
auricle {aur.) and anterior ventricle {vent.) is quite apparent even in fixed 
and mounted preparations. In fig. 15 (at the period of hatching of the 
embryo) the digestive system consists of a simple tube, the gut (^7.), ending 
in an apparently closed anus {a.) close behind the yolk-sac. At the 
anterior termination of this digestive tube the liver (ir.) is formed as a 
knob of cells projecting ventrally from the mid-gut part of the intestine. 
The liver, which at this stage is single-lobed, soon becomes bi-lobed, and 
the gall-bladder {g.h., fig. \^ et seq.) is apparent as a clear vesicle either 
towards the surface of the liver or between the lobes, when that organ 
becomes bi-lobed. With the increase of the young larva, there is an 
increase in size of the hepatic organ. 
The gut, which is a simple tube in fig. 15, exhibits an indication of 
further differentiation in figs. 16 and 17, and the corrugations of the 
walls of the intestine make their appearance. In fig. 1 8, when the larva 
is two and a half days old, the gut is distinctly divided into two parts, 
the hind-gut (h.g.) or rectal portion being constricted by a narrow neck 
from the mid -gut {m.g.) This constriction is seen at all subsequent stages, 
and the villous character of the mid-gut becomes more pronounced. The 
mid-gut in the larva of five days (fig. 19) begins to show a bending 
on itself, which in two or three days becomes a loop interposed between 
the position of the liver and the posterior half of the mid-gut. The an- 
terior part of the loop forms the stomach (,9^., fig. 21). An oesophageal 
portion {f.g.) of intestinal canal appears simultaneously with the bending 
of the raid -gut on itself, and is shown in figs. 19-21 ; the mouth (m.) is 
seen between the mandible {mn.) and the olfactory organ. The mouth 
when first seen is covered by a tissue layer stretching from the upper to 
the lower jaw. 
The urinary vesicle {u.v.) is seen posterior to the gut in the angle between 
