No.2.] DEVELOPMENT OF MANICINA AREOLATA. 205 
advanced. On comparing this side with Fig. 12, we see the 
cesophagus has moved away from the surface ectoderm, but 
while doing so has remained connected with it by a band of 
supporting lamella. Running through the series of sections, it 
is found that in this meridian also an ectoderm lobe has grown 
down from the cesophagus. This lobe is the primary filament. 
It is much wider at its start, near the cesophagus, than its fellow 
on the opposite side, but soon dwindles to about the same size. 
Like the cesophagus above it, this filament is in a more advanced 
condition than the filament on the right side. Since we know 
that the left side of the cesophagus itself has passed through the 
condition which exists on the right side, we are pretty safe in 
believing that the left filament has likewise passed through the 
condition in which the right filament is found. The lack of a 
fairly intermediate stage between Fig. 12 and the larva we are 
studying, is to a certain extent supplied by the larva from which 
Fig. 11 was taken. In this individual, which was sectioned 
transversely, the cesophagus extended so much farther down in 
the meridian of the first mesentery than round its general lip, 
that in this meridian it formed a very evident though not a very 
long lobe. This lobe, which becomes the primary filament, is 
shown in Fig. 11, and is to be found in the sections immediately 
below the one figured, growing smaller towards its end. 
On comparing the first and second filaments in Fig. 17, it is 
clear that the endoderm, which at a higher level grew in between 
the cesophagus and body ectoderm, has likewise forced its way 
beneath the primary filament, and thus given rise to the first 
mesentery. The mesentery is continued from the level of Fig. 
17 downwards, as a very slight endodermic ridge on which rests 
the filament. The axial band of supporting lamella is continu- 
ous with a thinner lamella, separating the filament from the 
endoderm. Immediately beneath the opening of the cesophagus 
the mesentery is much more elevated than at a lower level. 
The elevation of the mesentery in this region is connected with 
the first appearance of an intermesenterial chamber. 
In the larva we have been studying the first pair of intermes- 
enterial chambers has been marked off. Round the cesophagus, 
however, they are still solid, Fig. 14, though at a lower level 
the hollowing out of the solid endoderm has begun. At this 
level, Fig. 15, which is just beneath the opening of the cesopha- 
