No. 2.] DEVELOPMENT OF MANICINA AREOLATA. 209 
porting lamella between the cesophageal and surface ectoderm, 
The cesophagus now grows downward in this meridian as a lobe 
of ectoderm, which represents the primary filament, and which 
pushes the endoderm before it. On the opposite side of the ani- 
mal, along the line of the second mesentery, the cesophagus be- 
comes applied to the body ectoderm in the same manner, and a 
lobe grows down from it to form the second filament. The 
mesenteries as such are formed by the ingrowth of the endoderm 
between the body ectoderm and cesophagus above, and between 
the body ectoderm and the filaments below. The primary pair 
of intermesenterial chambers are at first solid. The larger cham- 
ber acquires its cavity before the smaller, the excavation travel- 
ling from the lip of the cesophagus upward, and from the first 
toward the second mesentery. The excavation of the primary 
chamber is closely followed by the reflection of ectoderm into 
this chamber, the reflected ectoderm running up the cesophageal 
wall, and driving the endoderm before it. The second pair of 
mesenteries appear in the larger chamber as longitudinal ridges 
of the supporting lamella, which cause no elevation of the endo- 
derm. 
V. HIsTOLoGY OF THE LARVA. 
It may now be advisable to describe the histology of the 
larve, going when necessary beyond the stages already studied. 
a. The Surface Ectoderm. 
The ectoderm at the time when the supporting lamella is 
formed, Figs. 5 and 6, consists of columnar cells, the protoplasm 
of which shows a great tendency to break into small polygonal 
balls. While the ectoderm is in this condition the mucus cells 
appear (m, Fig. 6, Figs. 7, 8), as pear-shaped bodies in the 
peripheral ends of the columnar cells. The contents of the 
pear-shaped body stains a deep blue with haematoxylin, and is 
thus distinctly marked off from the surrounding cells. It ap- 
pears to be fluid from the start, as even in such young stages as 
Fig. 7, many of the mucus cells have poured out their contents, 
which adheres to the mouth of the cell as a little mass of a blue 
color. This tendency of the mucus cells to eject their contents, 
presumably when the killing fluid touches the larva, is very 
