THE DEVELOPMENT OF SYMBRANCHUS MARMORATUS. 17 
they are not so strikingly conspicuous as in the younger 
stages. 
The increase of distance between the anterior point and 
yolk-mass is due not only to the lengthening of the gill- 
chamber^ but also to the formation of a snout, which is now 
quite clearly defined, and which has grown out from under 
the cerebral part of the brain. The telencephalon is in con- 
sequence now nearly at the same level as the optic lobes. 
This snout formation has the effect of carrying the eyes to a 
more anterior position : they now lie under the cerebral 
regions quite anterior to the optic lobes. The plane of the 
mouth opening becomes rotated upwards so as to assume a 
definitive horizontal position. 
The rostrum has almost disappeared. Chromatophores are 
increasing in number ; the creature is more opaque, so that 
liver and bladder are less conspicuous in the xylol specimens. 
The median dorsal fin in Stage 32 is dying away anteriorly. 
It begins to rise from the general surface behind the gill- 
chamber. 
Stage 33 (PI. 2, fig. 15). — Thirteen days after hatching 
the creature is about 21 mm. long and the yolk-sac has com- 
pletely disappeared. A little mound — striking, because of the 
absence of pigment, which by now is abundant all round it — 
is all that remains of the rostrum. The root of the pectoral 
fins has become shifted ventrally nearly to the mid-v^entral 
line. The lateral parts of the crescentic opercular opening of 
earlier stages are gradually being closed up, so that in this 
stage the opercular opening is quite ventral though relatively 
larger than in the fully adult condition. 
The posterior parts of the outer wall of the gill-chamber^ 
especially in the neighbourhood of the opening, are exceed- 
ingly thin. Normally the exit from the gill-chamber is closed 
and concealed, except when Avater is being actually expelled 
from it. The shoulder girdle and the attachment of the fins 
are now quite anterior in position to the bind end of the 
branchial chamber, the opercular flaps overlapping the base 
of the fins, which are now somewhat crumpled instead of 
VOL. 59, PART 1. NEW SERIES. 2 
