No.1.] THE DEVELOPMENT OF BALANOGLOSSUS. 45 
over its end is absorbed, and then absorption of the ectodermic 
cells at the end of the tube also takes place, so that a communi- 
cation is formed between the hollow of the body-cavity and 
the lumen of the collar-pores. The point of opening becomes 
later the funnel of the collar-pore. In Fig. 53 is drawn a portion 
of a cross-section at a stage when the tube of the collar- 
pore having pushed into the body-cavity is on the point of 
opening into the body-cavity by absorption of the end of the 
tube. 
This figure contains also other points of interest. Lying 
ventral to the nerve chord that is in process of forming, a 
small pair of body-cavities are cut. These are forward exten- 
sions from the last or third pair of body-cavities. At this 
stage they reach as far forward as the middle of the collar 
region. Numerous large mesenchyme cells are found in the 
upper portion of the section. 
Surface preparations show that the collar is distinctly marked 
off from the body. A zone of deeply stained nuclei is found 
_along its posterior border. 
As shown in Fig. 48, Pl. IV, the third body-cavities have 
filled in the space between the endodermal wall of the digestive 
tract and the body wall. Posteriorly each extends nearly to 
the end of the larva, and anteriorly we have seen each sends 
out a forward prolongation. The forward extensions start at 
the level of the collar-pores. The somatic wall of this pair of 
body cavities is twice as thick as the splanchnic layer, although 
the distinction is not seen in so small a figure. At the level 
of the posterior edge of the collar the third pair meet the 
second pair of body cavities. The second pair of body cavities 
was described in connection with the collar-pores. It has not 
opened out to the same extent as the last pair. 
The first pair of body-cavities, vzz., the proboscis-cavity and 
the proboscis vesicle, almost completely fill up the blastocoel- 
cavity of the proboscis. A portion of a cross-section of the 
proboscis is shown in Fig. 54. The mesoblastic wall of the 
body-cavity is almost in contact with the ectodermal body-wall 
(the space between is due to shrinkage). Projecting from the 
wall into the lumen of the cavity, are longitudinal furrows. 
