24 
MONICA TAYLOli. 
solid stalk conuecting liver and alimentary canal. The lujnen 
of the bile-duct develops from the alimentary canal outwards. 
In the next stage (27) the formation of the tongue has 
begun. Its adult relations are illustrated by PI. 3, fig. 22, 
there being nothing of special interest in its development. 
The gill-cavity is still small though the branchial apparatus 
has elongated considerably. The thymus rudiment fii-st occurs 
here. It develops on the outer dorsal side of the four branchial 
clefts, 2, 3, 4, and 5. 
The ventral pancreatic rudiments are solid outgrowths from 
the bile-duct. They meet the dorsal pancreatic rudiment so 
that at its hinder end the stomach is almost surrounded by 
pancreas. The adult pancreas would seem to be mainly the 
derivative of these ventral rudiments. The gall-bladder, 
originally solid, is being hollowed out. The spleen is very 
conspicuous. The temporary cloaca has become quite flattened 
out. 
Stage 29. — A median sagittal section at this stage shows, 
in the constriction marking the origin of the tongue, the 
rudiment of the thyroid which is a solid derivative of the floor 
of the pharynx. (In PI. 3, fig. 22 this gland is seen at Stage 34.) 
The mouth is not yet open, but the rest of the canal has a 
lumen. The bulbus lies directly under the second gill-arch. 
In Stage 29 there is the first trace of respiratory tissue. The 
two pore-like openings into the gill-chamber are shown in 
PL 4, fig. 23 — a transverse section through this stage. The 
pancreas is very much invaded by the mesencbymatous 
vascularised masses which constitute the spleen rudiment. 
The posterior part of the liver widens out so as to cover 
in the dorsal part of the yolk, both right and left. 
The alimentary canal has almost reached its adult structure 
iu Stage 30. The larva is now capable of feeding and makes 
frequent swallowing movements, the respiratory lamellae 
on the gill-arches being well developed. The heart lies 
posterior to the gill-chamber now (compare PI. 4, fig. 23), and 
an opening from the latter on the ventral and veutro-lateral 
surface of the creature has been burrowed out. This is con- 
