196 Observations on the Development ['^^ZSm^il^uZn'^S?' 
exposed on the yelk-bag. From tlie anterior is continued a large 
arterial trunk, which speedily divides into right and left, the first 
arterial arch ; that for the left side (h) passes upwards ; the other, for 
the right side, crosses underneath the head, reaches a similar position 
and pursues a similar course to that of the left arch. Just before 
turning along the upper edge of the branchial rudimental structure, 
each trunk gives off two branches (seen at c) ; the first, a small one, 
forwards towards the mouth (the orbito-nasal) ; the second, by far 
the larger, passes upwards, and supplies the head (the carotid). 
The diminished trunk makes a great and sudden turn, and runs along 
the upper edge of the branchial rudiment, and below the ear (d). 
Across the space for the branchias are seen stretching two mem- 
branous arches, which extend from the heart to the continuation 
of the first arterial arch. These membranous arches are for the 
support of a series of vessels, the branchial arteries, four in number 
(on each side), when complete ; the number of bony (as hereafter 
they are) arches, when complete, being five. Between these two 
rudimentary arches is seen a faint arterial current passing from 
the primitive vessel before division to the arterial trunk above, and 
entering it. At a later hour three spaces were seen, enclosing two 
blood-currents (as at e). These structures, when fully developed, 
form the gills, or breathing apparatus. The first arterial arch con- 
sists of the portion from the heart to c. After this point it forms, 
with its fellow of the opposite side, the aortic circle, extending from 
c to /, where it joins its fellow underneath the vertebral axis. From 
c, the half of the circle runs downwards, lying underneath a large 
vein (the cephalic or jugular), until this vein turns downwards to 
enter the auricular sinus, a sort of antechamber to the heart (g), 
where the venous current may be seen to cross over the arterial at 
right angles. From this last point the vessel curves slightly upwards 
and inwards, joins its fellow, and the conjoined vessel constitutes 
the aorta. This passes down along the body, being placed between 
the vertebral structures and the intestine, nearly to the end of the 
tail. From the whole length of the aorta are given off at regular 
intervals arteries that pass directly backwards to the dorsal border 
of the fish, apparently marking out, or running between, the seg- 
ments of the vertebral axis. (Fig. 25 : a, spinal cord ; h, segments 
of vertebral axis ; c, intestine ; d, aorta, giving off vessels.) At the 
dorsal border these vessels form a system of capillaries, from which 
return a number of veins similar in number and situation to the 
arteries, and which ultimately, by means of cross-currents, seen at 
m and reach the great venous canal lying below the aorta. This 
channel, the cardinal vein, commences near the extremity of the 
tail, there being one on each side ; the narrowed aorta terminating 
in them by a complete turn on itself (Fig. 39, h). The vein runs 
along directly underneath or below the aorta, as far as the point 
