10 
MONICA TAYLOU. 
The organ seems to be a prolongation of the transparent 
tissues surrounding the dorsal part of the braiig filled with a 
lymph-like fluid without any apparent cellular elements and 
walled in by two layers of epidermis, the outer of which is 
flattened and horny (PI. 1 , hg. 11). In the specimens with 
the debris there is a slight thickening of the lower epidermis • 
cells in the neighbourhood of the debris which may possibly 
represent a vestigial cement-gland, but it has not been found 
possible to demonstrate a break in the continuity of the outer 
horny hiyer, which fact tells against such an interpretation of 
the organ. The line of demarcation between the mesenchyme 
of the head and the fluid contents of the rostrum is very 
definite. 
There is a i-esemblance between the figures of the early 
developmental stages of the rostrum of Callorhy n chus ( 10 ) 
and that of Sy in branchus, and also a certain amount of 
resemblance between their structure in the early stages, for 
Schauinsland says that the rudiment of the rostrum of 
Callorhy n ch u s is a large thin-walled sac covered with 
ectoderm and later becoming filled with mesoderm — a descrip- 
tion which might apply in ]iart to the organ in Sym- 
b ranch ns. 
Large unicellular glands are very conspicuous objects in the 
ectoderm in sections stained in Heidenhaiu and eosin. They 
are most abundant in the epidermis of the fins, but occur also 
in the general epidermis, and are to be found, though sparsely, 
on the rostrum. These glands are evidently larval structures, 
for they gradually disappear as the adult stage is reached. 
J3y this stage the dorsal and ventral median fins are well 
established and there is a cloacal opening which appeared in 
Stage 24, so that it is now possible to distinguish the anterior 
limit of the true tail. In dorsal view the only parts of the 
brain visible are the optic lobes and rhombencephalon because 
of the cranial flexure. In side view the infundibulum and 
hypophysis are conspicuous ; olfactory organs and eyes have 
become more lateral in position. The heart is more clearly 
seen, the ear is still visible (in the xylol specimens) anterior 
