THE DEVELOPMENT OF SYMBEANCHUS MARMORATUS. 43 
and olfactory lobes are developing. The infundibulum is 
closely apposed to the floor of the hind brain, which latter is 
becoming very solid. In addition to the commissures already 
present the cerebellar commissure has now appeared, a torus 
longitudinalis is distinguishable, and the fibres of the optic 
nerve are connected up with the roof of the mid-brain, as is 
usual. 
There is no fundamental difference between the brain of 
Stage 34 (Text-fig. 4 f) and that of the adult. The brain is 
by now (PI. 4, fig. 28) very solid, the once large ventricles 
being almost obliterated. The paired olfactory lobes are 
conspicuous at the anterior end of tlie telencephalon, while 
ventrally the lateral lobes of the infundibulum have greatly 
increased in size. The cerebellum and valvula cerebelli are 
also much larger, while the tuberculum acusticum and the 
vagus lobes have almost obliterated the once large fourth 
ventricle. 
The brain dissected from an adult Symbranchus is charac- 
terised by a more perfect separation of the chief brain parts 
one from the other. These are very close together in Stage 
34. Another difference is that the optic lobes are not 
relatively so large as they were. The telencephalon, on the 
contrary, is larger, while the cerebellum and valvula cerebelli 
are much more important than they were at Stage 34. The 
sacculus also develops late. 
Cranial Nerves. 
The cranial nerves of the adult Symbranchus as well as 
their respective branches are quite typical. 
The eye-muscle nerves are small, like the eye itself, and 
the olfactory and optic nerves are long because of the 
general elongation of the head. 
The developmental history of the cranial nerves is, briefly, 
as follow : 
Stage 21. — The rudiments of the ganglia of the fifth, 
seventh and tenth nerves are visible. 
