No.1.] THE DEVELOPMENT OF BALANOGLOSSUS. 59 
A cross-section of the neck or narrow base of the proboscis 
is shown in Fig. 77. On the dorsal surface, and laterally, the 
ectoderm is much thickened with a large amount of punctated 
substance. The ectoderm of this region is directly continuous 
in the median dorsal line with the nerve chord. 
On the left side of the body, the ectoderm is pierced by the 
exit-tube of the first body-cavity. The centre of the section is 
filled by the notochord with its large central lumen. Around 
the cavity is a protoplasmic layer containing nuclei, and the 
outer portion of the notochord is formed of the clear vacuo- 
lated ends of these cells. Below the notochord is the collar-rod, 
that has been formed, without doubt, by a secretion from the 
notochordal cells. It is marked by concentric lines of light and 
dark shading, and these correspond, very probably to alternate 
periods of rest and activity of the endodermal cells. On each 
side of the rod, where it slightly bifurcates, lie two spaces 
nearly filled with large muscle-cells (0.c.2). The walls are 
continuous behind with the walls of the collar body-cavities, 
and represent forward lateral protrusions from the collar- 
cavities. The muscles serve probably as the retractors of 
the proboscis. 
Dorsal to the notochord there is in the median line a cavity 
bounded by mesoderm cells,—the cavity of the proboscis vesicle. 
A cross-section of one of the collar-pores is shown in Fig. 
78. At this stage, and also in the two preceding stages, the 
outer wall is turned into the interior of the tube. The cells of 
this invagination are not so high as are those of the remainder 
of the wall, and do not seem to be ciliated. Into the hollow 
of the invagination the wall of the body-cavity is pushed. The 
rest of the periphery of the organ is also surrounded by a 
layer of mesoderm. The inturned portion is on the latero- 
dorsal side of the tube, and the opening or nephrostome is at 
the inner end of the tube. The groove of invagination is con- 
tinued backward as far as the external opening of the tube. 
The structure is wonderfully like a glomerulus. 
The nervous system at this stage is larger than in the pre- 
ceding stages and contains more cells in cross section than in 
the last case. 
