16 
MONICA TAYLOR. 
are never visible on the exterior of Syni bran elms. The 
branchial chamber, which is gradually hollowed out as 
development proceeds, develops a connection with the 
exterior by means of a small pore on each side, microscopic 
in character, dorso-lateral in position. These openings lie 
behind the posterior edge of the operculum and are found 
in the early stages (up to Stage 29), when the pericardium is 
still ventral to the pharynx. At Stage 30 the heart has 
suffered a backward migration relative to the gill-chamber, 
and by this time an additional opening to the exterior from 
the latter has been burrowed out on the ventral side of the 
creature. This ventral opening* becomes continuous with the 
dorso-lateral pores on each side, thus forming a single crescent- 
shaped opercular opening, the horns of the crescent being 
posterior and dorso-lateral in position. If the ventral part of 
this opening could be obliterated then the opercular conditions 
of Sym branch us would resemble those of an ordinary 
Teleosteau, i.e. there would be two lateral openings. How- 
ever, it will be shown that, as development proceeds, the 
operculum continues to grow b?..ckwards, and as it does so it 
fuses with the body-wall, so that eventually a single median 
ventral branchial opening is the result. Co-extensive with 
this backward growth of the opei*culum is the lengthening 
of the branchial arches and of the whole gill-chamber. 
Stage 32 (PL 2, fig. 13). — About eight days after 
hatching the creature begins to assume something of its adult 
appeai*ance and to use its gills quite freely. It is about 
18 mm. long, having a tail of 5 mm. in length. The yolk- 
sac has almost lost its globular appearance, and the inclusion 
of the yolk in the abdominal cavity, already alluded to, has 
continued. The yolk enclosed in the body of the creature 
now extends to within 1 mm. of the anus. The body anterior 
to the heart is much elongated, the most anterior point being 
at a distance of 2*5 mm. from the 3^olk-mass. The pectoral fins 
have, however, not grown in proportion to the rest of the 
animal : they are about the same actual size as in Stage 30. 
Hence, on account of the general growth of the creature. 
