208 WILSON. [Vot. II. 
able that the reason why the ectoderm is reflected into the 
primary chamber at such an early date, is that the second pair 
of mesenteries will be formed in this chamber. The series of 
longitudinal sections, Figs. 20-23, illustrates the early reflection 
of ectoderm round the lip of the larger chamber. The larva 
from which this series was made was at a stage intermediate 
between Figs. 14 and 18. The supporting lamella of both of 
the first pair of mesenteries were complete, as in Fig. 18; but 
the intermesenterial chambers were far from perfect. The 
smaller chamber was entirely solid, and the cavity of the larger 
was only partially forméd. In spite of the difference in age, it 
will do to refer the planes of the longitudinal sections to Fig. 18. 
The section, Fig. 20, thus is taken through the mesenteries 1 and 
2, and the corresponding filaments. The next three figures 
represent a series of sections to the left of this plane, the one 
farthest to the left lying in the plane a—é of Fig. 18. Finally, 
the left-hand mesentery in Fig. 20 is the primary mesentery, 
I in Fig. 18. With this orientation it is seen, on glancing 
through the series 20-23, that only in the neighborhood of the 
primary mesentery has the intermesenteric chamber been hol- 
lowed out. In all the sections the endoderm is solid on the 
right side of the cesophagus. Now, in Fig. 23, the intermesen- 
teric chamber (to the left of cesophagus) is in the state in which 
it was formed. The ectoderm has not yet been reflected round 
the cesophageal lip. But in the sections nearer to the mesen- 
tery, Figs. 21 and 22, the ectoderm as been reflected, and has 
driven the endoderm before it. The reflection of ectoderm, as I 
have said, follows very closely the excavation of this chamber. 
As the latter proceeds from mesentery 1 to mesentery 2 (Fig. 
18), so does the former; and by the time the chamber is com- 
pletely established, the ectoderm is reflected all the way from I 
to 2, as Is seen in Fig. 18. 
In the larva which was used to show the earliest appearance 
of the primary intermesenteric chamber, Figs. 14-16, the re- 
flection of ectoderm had already begun to take place round one 
edge of the primary oesophageal lobe (Fig. 16, left). 
To sum up the more important events described in this 
section : 
To form the first mesentery the whole cesophagus moves lat- 
erally, until in the meridian of the mesentery there is only sup- 
