THE DEVELOPMENT OP SYMBRANCHUS MARMORATUS. 13 
The living larva makes frequent swallowing movements and 
the gill-lamellae are differentiating. Thus preparations for 
branchial respiration are afoot. Hitherto respiration has 
been effected mainly by the pectoral fins. The larva has also 
begun to feed^ its food under artificial conditions consisting of 
earthworms and liver. In the living specimen dorsal aorta, 
posterior cardinal, caudal, subintestinal veins are well marked, 
while the ventral aorta and the four aortic arches can be 
seen. Tdie urinary bladder, the widened out posterior parts of 
the fused archinephric ducts, first visible in Stage 28, is con- 
spicuous in the xylol specimens of this stage. It now measures 
1 mm. in length and lies dorsal to the rectum, opening to the 
exterior immediately posterior to the anus, the cloaca having 
flattened out. 
Description of the Pectoral Fins. — It has been shown 
that in the early stages, 18 to 21, the pectoral fins consist 
of mesoderm masses — the ventral processes of certain 
anterior myotomes — covered by a two-layered epidermis. 
Very quickly, however, these masses begin to differentiate. 
Cartilage derived from the cells forming the central part 
of the fin-rudiment appears at Stage 24 to form the 
skeleton, which is visible externally, and the muscle-fibres 
are differentiated. The main mass of the fins, however, is 
mesenchymatous, the increase in size being mainly due to 
increase of this tissue. The fins are very transparent and 
beautiful objects as well as conspicuous by their large size, 
the longest diameter at Stage 30 being 2’5 mm. When 
living* they are quite red because of the rich network 
of capillaries. They are roughly semicircular in shape, 
the diameter of the circle lying nearest to the yolk-sac. 
The cartilaginous part of the fin is triangular (PI. 1, 
fig. 10 and PI. 2, fig 12), the apex of the triangle consisting 
of a rounded knob, to which most of the muscles are 
attached. This rounded knob fits into the shoulder girdle, 
dividing the latter into a dorso-lateral and ventro-lateral 
portion. The shoulder-girdle is little more than a thin 
lamella of bone surrounded by much muscle. The cartilage 
