204 WILSON. (VoL. II. 
down in this meridian. This is proved by Figs. 9 and Io, 
transverse sections from a larva in which the cesophageal move- 
ment had gone farther than in Fig. 8. Fig. g is about at the 
level a in Fig. 8, and Fig. 10 is at the level 4. In this larva 
then the cesophagus was closely pressed against the ectoderm at 
the level a, but was separated from it by endoderm at a lower 
level 6. In the later stage, Fig. 11, the lowest part of the 
cesophagus has completed the journey. The meridian in which 
the oesophagus is thus pressed against the ectoderm, is that of 
the first mesentery. (Though Figs. 9 and 10 are from the same 
larva, both the supporting lamella and endoderm are much 
farther advanced in the lower section than in the upper.) 
The larva from which the longitudinal sections, Figs. 12 and 
13, were cut, was very backward in forming the permanent 
endoderm. Fig. 12 is taken through the line a and 6 in Fig. 
II, and is in the plane of the first pair of mesenteries. It is 
only on the left side that the cesophagus is in contact with the 
body ectoderm. But the right side is following suit, and in a 
slightly older stage, Fig. 12, is in the same manner applied to 
the surface ectoderm over a narrow tract. Fig. 13 is to one side 
of aand 6 in Fig. 11, and is consequently out of the plane of 
the mesenteries. 
My next stage after Fig. 12, is the larva from which the 
series of transverse sections, Figs. 14 to 17, was taken. It is 
this stage which proves the meridians, along which the cesophagus 
is applied to the body ectoderm, to be really those of the first 
pair of mesenteries. Fig. 14 is the uppermost of the series, and 
is through the body of the cesophagus, the lumen of which is 
exceptionally large. On the right side the cesophagus is sepa- 
rated by jelly alone from the surface ectoderm. Following down 
the series of sections, we see that in this meridian the cesopha- 
geal ectoderm sends down a slender lobe, which, like the cesoph- 
agus above it, divides the endoderm and rests on an accu- 
mulation of jelly (right side of Figs. 15, 17). This lobe is a 
mesenterial filament. It is considerably shorter than the fila- 
ment on the opposite side, Fig. 17, and is probably the second 
one of the first pair. The right side of this larva is practically 
in the condition shown in Fig. 12, except that the cesophagus 
has formed a filament. 
On the left side of the larva, Fig. 14, matters are more 
