218 WILSON. [Vot. Il. 
is noticed in a good many specimens. In Fig. 37 the left half 
of the section, through an intermesenteric chamber, exhibits 
this phenomenon. The right half of the section is through one 
of the second pair of mesenteries. 
The fifth and sixth pairs of mesenteries appear simultaneously, 
but it is convenient to speak of one pair as the fifth and the 
other as the sixth. They are still very small in my oldest larval 
stage. The filaments for the fifth pair are probably formed 
from the lateral portions of the lobe x, Fig. 39, after it has been 
divided by the completion of the third pair of mesenteries. 
The filaments for the sixth pair, it seems, will be formed from 
the tracts of ectoderm, which belong to the chambers a and 4, 
Fig. 39. These tracts, it will be remembered, were in most 
larvee pushed completely back to the free edge of the cesopha- 
gus, where the second pair of mesenteries become complete. 
In stages with twelve mesenteries, however, such as Fig. 39, 
they have again appeared, though they usually extend but a 
very short distance above the lip. In a couple of larve as old 
as Fig. 39, they were unusually well developed, reaching as far 
up as the tract for the chamber c. 
In summing up the facts of the reflection of ectoderm it will 
be convenient to refer to Fig. 39. 
The ectoderm reflected into the larger of the two primary 
chambers is pushed down by the growth of the second pair of 
mesenteries. From it are formed the filaments for these mesen- 
teries, while the remainder of the original tract splits into three 
divisions. The middle division, chamber c, is not pushed en- 
, tirely to the edge of the cesophagus ; later in life, when the 
fourth pair of mesenteries is well developed, this tract grows 
once more nearly to the upper limit of the cesophagus. The 
lateral divisions a and 6 are pushed to the edge, but after the 
sixth pair of mesenteries has appeared they begin to grow up 
again. When the mesenteries of the third pair are well ad- 
vanced, the ectoderm is reflected into the smaller of the two 
primary chambers, and runs up the cesophageal wall nearly to 
the top of the chamber. The mesenteries, when they begin to 
grow down, carry a part of the ectoderm along their free edges 
as very slender filaments. The growth of the various tracts of 
reflected ectoderm is thus seen to follow in general the order 
of development of the mesenteries. 
