206 WILSON. [Vot. II. 
gus, the endoderm on one side of the first mesentery exhibits a 
small cavity. The cavity, when traced through the series of 
sections, is found to open into the ccelom in Fig. 16, forming, 
as it does so, the cave or bay which underlies one-half the 
primary filament in the figure. It is thus (comp. Figs. 14 and 
15) in the larger of the two primary chambers that the excava- 
tion of the solid endoderm begins, and, as I learn from other 
series, the excavation of this chamber is nearly completed 
before that of the smaller begins. The excavation starts, as we 
have just seen, in the immediate neighborhood of the first mes- 
entery. From this spot it gradually extends across the chamber 
to the second mesentery, travelling all the while from the lip of 
the cesophagus upwards. 
The second mesentery, which exists in its embryonic condition 
in Fig. 14, follows the example of the first. In Fig. 18 it is 
completely formed, and in the series of longitudinal sections, 
Figs. 20-23, which I shall describe later, the second mesentery 
is in the condition in which the primary is in Fig. 14; z.e., in the 
immediate neighborhood of the second mesentery the larger of 
the two intermesenteric chambers is solid, while the smaller 
chamber is entirely solid. 
In the larva from which the transverse section, Fig. 18, was 
made, the various processes which have been described are now 
completed. The cesophagus is swung by two complete mesen- 
teries. Both intermesenteric chambers—the larger on the 
left, the smaller on the right — have been hollowed out. Below 
the cesophagus, the mesenteric ridges extend the whole length 
of the larva, and the first pair of filaments about half the length. 
The section is slightly complicated by other features, of which I 
shall now speak. 
In the larger of the two chambers, Fig. 18, the second pair of 
mesenteries, 3 and 4, has appeared. The axial bands of sup- 
porting lamella as yet cause no elevation of the endoderm, and 
at a level slightly below the cesophagus are entirely lost. In 
Fig. 18 a, above Fig. 18, the axial band numbered 4 stretches 
across to the cesophagus, and still higher up 3 does likewise. 
In this section the position of the cesophagus is eccentric. This 
is very often the case at the extreme upper limit (the appearance 
is not due to oblique sections), and hence one mesentery usually 
runs out before its fellow (2 before 1). The mode of origin of 
