42 MORGAN. [Vou. IX. 
The second and third body-cavities have distinct cavities 
surrounded by thin walls, the posterior pair being the larger. 
The cells of the walls show a differentiation into fibres arranged 
longitudinally along the wall, z. ¢., parallel to the greater diam- 
eter (length) of the cavities. 
A new pair of organs appears at this stage, or nearer, per- 
haps, to a stage between that of Fig. 5 and Fig. 6, Pl.I. These 
are the collar-pores that arise as a pair of invaginations of ecto- 
derm, one on each side of the middle line just behind the edge 
of the collar. The earliest stage in the development of these 
that I have seen is shown in Fig 42. The inturned portion of 
ectoderm is the anlage of the organ. The later stages we will 
study in older larvae. 
Two of the mesenchyme cells of the blastocoel are drawn in 
Fig. 43. Each has a clear vacuole-like interior surrounded by 
a granular wall, and from the wall are sent out long pseudo- 
podial-like processes. Often two nuclei are found in a single 
cell, as seen in one of the figures. 
Surface views of the apical plate show that the eyes are 
undergoing important changes. Fig. 444 is a surface view of 
an eye and Fig. 44d is an optical section of the same. The 
outer line marks the limit of the pigment, but the division in 
the preparation is not a very sharp one. The crescentic line 
in Fig. 44 a, inside one end of the eye, marks the upper border 
of a hole, that leads into an invagination. In Fig. 44d we have 
a view obtained by a deeper focus of the lens. The invagina- 
tions seen in the preceding figure leads into a rather large and 
very irregular cavity bounded by the outer clear ends of the 
eye cells. The outline of the clear zone is indicated by the 
middle line of the figure, and the clear zone itself marked by 
cross lines. The outer zone between the middle line and the 
outer (convex) line forms a dense border of pigment. The 
invagination of the eye is even more complicated, for on focus- 
sing at a deeper level the tube of the invagination sinks deeper 
into the bulb of the eye. 
In Fig. 45 is drawn a cross-section of the eyes, taken from 
a longitudinal-horizontal section of the embryo. One eye 
(on the right) is cut at the opening of the pit. The other 
