No.1.] ZHE DEVELOPMENT OF BALANOGLOSSUS. Ay 
The digestive tract is short with thick walls. The oesoph- 
agus runs immediately posteriorly from the mouth. In its 
dorso-lateral walls two pairs (and the beginning of a third pair) 
of gill-pouches are found. In Fig. 57 a side view of the 
oesophagus is shown. Three pairs of gill-pouches are seen 
projecting from the dorso-lateral walls. The first or second 
pairs have their median dorsal wall turned back into the cavity 
of the protrusion and these form subsequently the tongue 
bars. 
The constriction separating the stomach from the intestine is 
disappearing. It simply pulls apart to form the lateral walls of 
the stomach-intestine of that region. 
Between the last stage (Fig. 6) and the next stage (Fig. 7, 
Pl. I) a number of intermediate stages have been cut and 
studied. During this period two important changes have taken 
place, vzz., in the collar-pores and in the formation of the gill- 
slits. In other respects except an increase in size the larvae 
have undergone very little change. 
In Fig. 59 is drawn a portion of a longitudinal (dorso-ventral) 
section of a stage a little older than drawn in Fig. 52. The 
section passes through the ectodermal opening of the invagina- 
tion that gives rise to the collar-pore. The upper portion of the 
figure is on the collar side —the invagination lies immediately 
behind the collar. The bottom of the invagination is flattened 
out against the first gill-pouch. A double wall, the outer layer 
of ectoderm and the inner of endoderm, separates the cavity of 
the digestive tract from the lumen of the invagination. The 
ectoderm at the point of contact is much thinner than the ecto- 
derm elsewhere and shows signs of degeneration. The endo- 
dermal wall is also thinner over the region of contact. The 
position of the funnel of the collar-pores is shown at f. The 
section does not pass through the lumen of the collar-pore 
which turns forward from the main invagination and which 
later. becomes the collar-pore proper, but the position of the 
tube is indicated by its wall extending into the body-cavity. 
A series of three sections through the region of the collar- 
pores at a stage older than the last are shown in Figs. 60, 61 
