THE DEVELOPMENT OF SYMBRANCHUS MARMORATIJS. 35 
by the subintestinal vein, a large vein which is situated in the 
ventral part of the mesenchymatous tissue underlying the ali- 
mentary canals (-9. i. v., PL 4, fig. 25). This vein first 
appears in that part of the tail ’’ which is nearest the yolk- 
sac, and gradually extends backwards as the tail ” elongates. 
Stage 24. — The straight tubular heart is twisting and the 
rudiment of the bulbus first appears. The ventral aorta is 
exceedingly short and topographically posterior to the 
ventricle, on account of the position of the pericardial rudi- 
ment. (PI. 1, fig. 11, shows the ventricle.) The vessels in 
the branchial arches are making their appearance, though not 
yet connected up with the ventral aorta. 
The anterior cardinals, ducts of Cuvier, and veins of the 
fin are appearing, in the form of clear spaces in the mes- 
enchyme provided in some cases with blood-corpuscles, while 
in the tail a caudal vein is visible. This caudal vein some 
little distance anterior to the anus divides up into two.branches, 
one of which forms the inter-renal vein, and dies out anteriorly 
at this stage ; the other, descending to the right of the ali- 
mentary canal, forms the subintestinal vein alread}^ mentioned 
(.s. i. V. PI. 1, figs. 9, 10, and PI. 2, fig. 13). This sub- 
intestinal vein breaks up in the yolk into a number of capil- 
laries which drain into two large vitelline veins, which in turn 
form by their union the sinus venosus. 
Two large vessels are given off to the pectoral fins from 
the dorsal aorta in the neighbourhood of the pronephros. 
Posterior to the pronephros, the dorsal aortge, until now dis- 
tinct, are fusing up to form a single vessel. 
Stages 25 and 26 (Text-fig. 2). — The bulbus lies im- 
mediately ventral to the first branchial arch, so that the first 
branchial artery has a vertical direction, and the ventral aorta 
must take a posterior direction in order to supply the three 
other branchial arches. This backwardly directed part of the 
ventral aorta is paired. A smaller branch from the bulbus, 
the remnant of the disappearing hyoid aortic arch, runs for- 
ward to the hyoid arch, but it is quite insignificant compared 
with the four aortic arches, which practically convey all the 
