216 WILSON. (Vou. II. 
carrying down at the same time the endoderm between a and 4. 
Thus came the condition shown in Figs. 30 and 31. When the 
time came for the mesenteries as such to be formed, the endo- 
derm which had been pushed down between a and 6 was com- 
pelled to grow up again, becoming excavated so as to form the 
intermesenteric chamber. It is in this process that we see the 
endoderm in Figs. 32 and 33. The irregularity of the larva, it 
will be noticed, is confined to one side: the right side of the 
sections is normal, and doubtless the other member of the sec- 
ond pair of mesenteries would have developed in the usual way. 
The larva just described was the only young specimen I 
observed, which exhibited this peculiar variation. I found a 
few older individuals, however, in which one of the filaments of 
the first pair was so intimately connected with one of the sec- 
ond pair, as to render it probable that the two were simulta- 
neously formed from a common lobe in the manner shown in 
Figs. 30-33. The larva, Figs. 27-29, to which I have already 
referred several times, comes under this head. Though the 
section, Fig. 29, is an appreciable distance below the cesopha- 
gus, the filaments 1 and 3 are still united, and bear evidence of 
their common origin. The other mesentery of the second pair, 
4 in Fig. 28, has itself been formed, and is gaining its filament, 
in the normal manner. The reflected ectoderm only extends 
from 2 to 3, and if the latter mesentery has been formed in the 
manner suggested, the ectoderm never was reflected between 
3 and I. 
Returning to the normal development we have now to trace 
the origin of the filaments for the third pair of mesenteries. 
These filaments are derived from a lobe of ectoderm, which is 
reflected into the smaller of the two primary chambers. The 
reflection takes place after the larvee have become attached. In 
a stage with twelve mesenteries, Fig. 39, the lobe is marked ~. 
The section is taken at a level higher than that reached by the 
lobe belonging to the chamber c (marked #.Z. in Fig. 27). In 
a section just above Fig. 39 the third pair of mesenteries bury 
themselves in the lobe z, much as 4 does in Fig. 28. In the 
uppermost sections, where the mesenteries are complete, x is 
not found. After a great many trials I succeeded in getting a 
radial section through one of this pair of mesenteries, which 
had forced down the reflected ectoderm so that the latter lay 
