No.1.) ZHE DEVELOPMENT OF BALANOGLOSSUS. 61 
the two walls has a much greater volume. The blastocoel is 
filled with a gelatinous fluid. This fluid necessarily increases in 
quantity but does not necessarily retain the same consistency. 
I have no observations to show whether the gelatinous fluid 
becomes diluted as development progresses or not. We might 
suppose that additional water is taken in so that the ecto- 
dermal wall becomes stretched, or we might suppose — in 
the absence of data 
The latter view seems much the more probable, for so far as 
that the amount of the jelly increases. 
microscopic examination of dead material goes it shows that 
the blastocoel fluid is much denser in the older stages. I there- 
fore assume that the fluid is formed by cells of the larvae, but 
whether these cells are ectoderm, mesenchyme or endoderm I 
do not know. 
The ectodermal ciliated bands increase in size and in the 
number of their cells and the region of the apical plate is from 
the start thicker than the general surface ectoderm. 
There is a zone of thickened ectoderm found in the earliest 
stage just in front of the large circular band. This is an 
important point for, as we have seen, it is in this region and 
from these cells that the collar region, the gill region and the 
anterior half of the body proper subsequently develop. 
In the very early stages the posterior plate, stretching hori. 
zontally across the posterior end of the animal, is a thin mem- 
brane. It has thickened when the larva has reached Stage 3 
and before the next Stage (4) has become as thick as the ecto- 
derm anterior to the circular band. From this posterior plate 
is developed the lateral walls of the posterior half of the larva. 
We may now pass to the second phase of larval growth dur- 
ing which a continuous decrease in size takes place. During 
this period the proportions of the body change very consider- 
ably, but z¢ zs znteresting to note that no new organs are formed 
while the larva ts decreasing in size. 
As the size of the larva decreases the ectoderm increases in 
thickness and although these two changes go on together it 
must be remembered that the ectoderm continues to thicken 
after the larva has gone into the sand and when it is again in- 
creasing in size, The blastocoel decreases greatly in volume, 
