THE DEVELOPMENT OF SYM BRAND HUS MARMORATUS. 45 
stage. The nuclei, which later on will form the nerve-sheath, 
lie more or less scattered round this protoplasmic-like develop- 
ing nerve. 
In Stage 26 these nerve-sheath nuclei, which have been 
derived from the mesenchyme separating spinal cord from 
myotome, are more closely apposed to the nerve-rudiment, 
which is now much more strongly fibrous and has almost 
entirely lost its protoplasmic character. In stages beyond 
26 the nerves have the usual adult structure. 
IV. Summary. 
(a) General Features of Development. 
(1) The egg of Symbranchus is small, its development 
typically Teleostean and rapid, the larva hatching out in 
about seven days at a tropical temperature. 
(2) A rostrum appears just before the larva hatches, 
increases in size, attains a maximum length of about 1 umi. 
when the creature is 7 mm. long, decreases in size, gradually 
dying down to a rounded pad, and eventually disappears just 
before the adult stage is reached. 
(3) The larva possesses pectoral fins and the shoulder 
girdle persists in the adult. These fins appear early, are 
nmscularised by the first three trunk myotonies and innervated 
by the first three spinal nerves. They develop rapidly, reach 
their maximum size seven or eight days after hatching, shrivel 
somewhat, and then drop off bodily at Stage 34. The 
pectoral fins are mainly respiratory organs and possess a rich 
network of capillaries. There are three principal blood- 
streams in the fins — one central, afferent, two marginal, 
efferent. 
The establishment of perfect branchial respiration is co- 
incident with the falling off of the fins, i. e. when the creature 
is ten days old. 
(4) No trace of pelvic fins has been found. 
(5) Perforated gill-slits of the Elasmobranch type do not 
occur in early stages, the clefts only becoming perforate after 
