No.1.) THE DEVELOPMENT OF BALANOGLOSSUS. 29 
plate, but to the ventral side, z.¢., nearer to one free end of 
the band than to the other. The structure of the eye is not 
easily made out. A crescent of clear cells is found, surrounded 
on the more central side (convexity of the crescent) by pig- 
mented cells. Cross and longitudinal section of the larva show 
that all these parts of the eye lie in the surface of the ectoderm. 
The structure of the digestive tract, as shown by serial 
sections is the same as in the preceding stage. I have added 
two figures for the sake of orientation. Fig. 8 cuts the diges- 
tive tract above the entrance of the tube, 70, leading to the 
true mouth, while Fig. 9 shows the external opening of this 
tube. The oesophagus does not open immediately to the ex- 
terior by the mouth opening, but into a depression of the sur- 
face ectoderm formed by the tube. Along this tube, 7d, run 
the anterior and posterior limbs of the circum-oral band, cut in 
four places in the figure. The two bands on the inner side 
unite just below the mouth, those of the outer side unite above 
the mouth. These ciliated cells are directly continuous at the 
mouth with the ciliated cells of the oesophagus. The stomach 
is nearly circular in outline, somewhat flattened on the ventral 
side. Here the wall is thickened to form the ventral ciliated 
ridge of the stomach. On each side of the stomach are seen 
the horns of the first body-cavity, and on the dorsal side of the 
section the tube formed by this body-cavity is cut across. In 
structure the anterior body-cavity is the same as in the last 
stage, but larger, and the solid prolongation to the apical plate 
is hollowing out ; the lumen is also extending further into the 
lateral horns. 
At this stage the proboscis vesicle appears. It is important 
that the origin of this organ should be accurately known, and 
I have paid a great deal of attention to the beginning of the 
structure. The earliest stage that I have obtained is shown in 
Fig. 10. A portion of the ectodermal wall is shown in the 
lower part of the figure. The anterior (or proboscis) body 
cavity is seen at dc’, and the stomach wall above. Between 
the ectodermal body wall and the mesodermal wall of the body- 
cavity is found a collection of what seems to be mesenchyme 
cells. These are attached on one side to the ectoderm by 
