144 
MR. W. K. PARKER OK THE STRUCTURE AKB 
behind the anus (an.); it is the tail, and the intestinal opening is but a small dis¬ 
tance behind the huge enlargement caused by the yolk-mass; that mass is three- 
fifths the length of the embryo. A vertical line drawn through the hind-brain and 
eye-ball would bisect a special enlargement in front of the yolk-mass. This enlarge¬ 
ment, caused by the heart (h.), would be partly in front of that line, and looking 
at the embryo from the upper end of the line we should see the heart lying in front 
of the head. 
The liver ( l .) would be behind the lower end of the line, for most of the yolk-mass 
which is included in the rudimentary stomach lies in front of the liver , contrary 
to what takes place in the Lamprey, Frog, Elasmobranch, and the Amniota (Balfour, 
op. cit., p. 90, fig. 56). According to that excellent observer “ the peculiar flattening 
out of the embryo on the yolk is due to the mode in which the yolk becomes 
enveloped by the hypoblast ” (p. 91). 
In an embryo more developed, but slightly smaller (Plate 12, figs. 2, 3, 6^ millims. 
long) than that figured by Mr. Balfour (his fig. 53 was 7 millims. long), the yolk-mass 
is only one-third the length of the embryo—in the last it w T as half as long—and the tail 
is now also about one-third the length of the whole larva. The head is now fairly free 
from the yolk-mass, at least as far as to the hyoid fold (%.), but the heart (h.) is a very 
short distance behind the mouth ( m .), and the end of the intestine (an.) is seen to be 
three times as far from the yolk-mass (y.). The tail is at present perfectly diphycercal; 
the notochord running along the middle of the upturned tail, and the azygous fin is 
equally above and below the axis. 
The mouth (fig. 3, m.) is a crescentic slit, with a small anterior bay in the middle of 
its anterior convex margin, and with a definite labial rim. In front of the oral opening 
and under the fore-brain (C 1 .), behind a transverse ridge that passes from side to side 
parallel with the mouth, there are two pairs of pupiform enlargements with their 
narrow ends behind, and those of each pair approximated; these are the rudiments of 
the barbels (bb.). There are four visceral folds with their clefts, the two anterior 
larger folds being the mandibular and the hyoid; from the latter a free edge is growing, 
the opercular fold (op.) ; between the two bars the first (or hyomandibular) cleft (cl 1 .) is 
closed ventrally, already, its permanent uppermost part will be the spiracle. The 
convex front lip of that limited cleft is thickening inside, and tends to produce a 
mandibular gill. The nasal sac (ol.) is still a shallow" pouch with a thickened rim, the 
outer coats of the eye (e.) are closing over the lens, and the auditory sac (au.) is a 
smallish oval mass showing no evident involution. 
In this larger embryo the basis-cranii and visceral arches are still composed ol 
<£ embryonic cartilage; ” these skeletal structures can be better studied in somewhat 
more advanced larvae. 
