NOt THE DEVELOPMENT OF, BALANOGLOSSUS. 35 
present stage. In the later stages, as we shall see, the in- 
vagination becomes deeper and much more extensive. 
To return to our study of the larva. Surface views of the 
posterior plate are not appreciably different from those of the 
extra-oral area. A portion of the posterior plate not magnified 
very highly (Zesss 2, AA) is. shown .in Mig, 23,-Pl. TV... The 
periphery of the plate is bounded by the circular ciliated band 
(shown more in detail in a later figure). The portion of the 
plate drawn corresponds to the side behind the median dorsal 
line. Within the plate the outlines of the third pair of body- 
cavities (0.c.3) are seen; these do not meet in the middle line. 
Concentric with the posterior ciliated band is a much smaller 
band, a second circular band, and in reality the more posterior 
of the two. Comparing the position of this band with what 
we found in earlier stages we see that the distance between 
the two bands is greater and that the smaller band itself is 
narrower and more sharply defined. A figure of the band 
drawn to a larger scale is shown in Fig. 24. The nuclei along 
the course of the band are elongated and stain more deeply 
than those of the plate; moreover, the band is clearly defined 
owing to the elongation of its cells in the direction of the 
course of the band. 
The large circular ciliated-band is shown in Fig. 25. The 
upper portion of the figure is part of the extra-oral area in front 
of the band, and that below corresponds to the posterior plate. 
The band itself is composed of large cells that give rise to 
the large flagella-like cilia—these having fallen off in the 
preparation. Behind the band (below in the figure) is found 
a line of deep-stained nuclei that I should expect to carry cilia 
during life. Behind this lies the posterior plate with its few 
gland-cells. The region anterior to the circular band is thicker 
than the rest of the ectoderm (except along the bands) and 
contains, as seen in the figure, numerous gland-cells. 
Cross-sections of the larva have in the upper half an hex- 
agonal outline, due to the six areas between the anterior band. 
A portion of a cross-section, passing below the level of the 
mouth, is drawn in Fig. 26. The wall of the stomach is 
thicker than in the preceding stage and has a relatively smaller 
