No.1.] HE DEVELOPMENT OF BALANOGLOSSUS. 5a 
body-cavity, and this extension is in close contact with the 
median forward extension of the third pair. Between the 
ventral wall of the proboscis vesicle and the dorsal wall of 
the oesophagus is a large blastocoel space filled with fluid. 
This space is continuous with the large space between the 
walls of the proboscis vesicle and proboscis body-cavity. 
Two cross sections of the nerve-cord are shown in Fig. 
65 and Fig. 66. They are from different embryos of about 
the same stage. In the first of these the nerve-cord has 
pinched off from the ectoderm above, but is still in contact 
with it. The large lumen of the cord is arched over by 
cells that have grown in from the sides of the cord. In 
the second figure the cord has sunken below the ectoderm 
of the collar, but retains a connection with the latter by a 
narrow line of ectodermal cells. The lumen of the cord is 
conspicuous, and the number of nuclei in cross section is 
gradually diminishing, as may be seen by comparison with 
the previous figures. On each side of the nerve-cord lie 
the collar body-cavities, but as yet they have not come into 
contact at the sides with the nervous system. 
This stage is marked by the presence of two pairs of gill- 
slits opening at the surface of the body. Serial sections show 
the process of formation of the second pair of slits. The 
second pair of pouches protrude to meet the ectodermal in- 
growth that gave rise in the early stages to the collar-pores, 
and subsequently served for the first gill-slit as well. The 
new point of contact is immediately behind the opening of 
the first gill-slit. The walls fuse and separate. The relative 
position of the openings is shown in Figs. 67 and 68, Pl. VI. 
The figures are portions of longitudinal (dorso-ventral) sec- 
tions. The first is nearer the side of the body. The collar- 
pores and opening of the first gill are shown (g!). The collar- 
pore is to be spoken of now as limited to the tube leading 
from the internal opening into the body-cavity to the external 
opening at the edge of the gill-slit. In Fig. 68 the opening 
of the second gill-slit is shown at ¢2. Jt opens at the bottom 
of the same pit that is the common exit of the first slit and 
collar-pore. The depth of this pit is without doubt subject 
