THE DEVELOPMENT OF SYMBRANCHUS MARMORATUS. 37 
mass bathes the tissue of the liver, which, as regards its cir- 
culation, may be considered as part of the yolk. The large veins 
from the fin become connected up with the ductus Cuvieri. 
Stage 26. — The hyoid aortic arch has almost disappeared. 
The four branchial aortic arches are large. The heart is 
becoming relatively more posterior and more twisted. There 
are many connections between the inter-renal vein and the sub- 
intestinal vein, which do not show any segmental arrangement. 
Inferior jugulars now appear, and the right posterior cardinal 
is becoming larger than the left one. 
Stages 27 and 28. — The heart is now in such a position 
that the bulbus lies under the third gill-cleft, so that the 
ventral aorta has to run forwards to the first and second 
afferent branchial arteries and backwards to the third and 
fourth. A small branch of the ventral aorta to the hyoid 
arch still persists. In Stage 30 this has disappeared. The 
aortic roots join up to form the dorsal aorta posterior to the 
fourth branchial arch, the fourth branchial efferents being 
almost at right angles to the dorsal aorta. Anteriorly the 
aortic roots are continued forwards as the dorsal carotids, 
which receive the efferent end of the hyoid aortic arches. 
Near the infundibulum the carotids divide up into internal 
and external branches, which are distributed, the former to 
the brain, the latter mainly to the eyes and nose. The 
inferior jugulars which collect the blood from the lower jaw 
and the parts below the ventral aorta, and which are asym- 
metrical, the right being smaller than the left, pass between 
the bifurcating anterior ends of the thyroid. The ventricle is 
now topographically the anterior part of the heart. 
Stage 29. — Text-fig. 3a shows the principal veins at this 
stage and needs no description. 
Stage 30. — The vascular system is practically in its adult 
condition at this stage. Sinus venosus, atrium, ventricle and 
bulbus have assumed their adult topographical relations, 
though all these structures are not relatively so elongated as 
in the adult Symbranchus, nor does the heart occupy such a 
posterior position. The ventral aorta, moreover, is not the 
