44 MORGAN. [Vor. IX. 
tion of the degenerating tissues. That they are modified 
mesenchyme cells, I think, there can be no doubt ; but they are 
immensely larger than the mesenchyme cells, as may be seen 
in the figure. 
The most important change that is now going on in the 
ectoderm is the formation of the dorsal nerve-cord, or central 
nervous system. Fig, 49 is taken from the posterior end of 
the collar region, —a little in front of the point where the 
collar-pores are invaginated. A central plate of ectoderm 
is in process of sinking beneath the surface. Its nuclei are 
more abundant than the nuclei of the rest of the ectoderm, 
and the plate as a whole stains more darkly than the rest of 
the ectoderm, of which it is still a part. There is in the 
center of the plate quite a collection of punctated substance 
continuous at each side by a narrow zone with the punctated 
layer of the ectoderm. At the sides of the plate the ectoderm 
is arching over. 
Fig. 50, Pl. V, is the sixth section in front of the last; four 
more sections forward and the ectoderm is continuous with 
the ectoderm of the proboscis. In this figure the ectoderm has 
met above the nerve-chord from side to side. This represents 
the condition of the greater number of sections, and nowhere 
as yet is the chord pinched off from the ectoderm. 
A second circular band presumably ciliated is found in the 
middle of the region between the large band and the end of the 
larva. This is the same band that was found encircling the 
posterior plate of earlier stages. A small portion of the ecto- 
derm in the region of this band is drawn in Fig. 51. The line, 
generally double, of deeply stained nuclei marks the course of 
the band. On each side the vacuolated cells of the ectoderm, 
with scattered nuclei, are to be seen. 
The collar-pores have reached a later stage of development 
at this time. The earliest stage we saw in Fig. 42 and a later 
stage is drawn in Fig. 52. We find in the older stages that 
the invagination of ectoderm is applied to the posterior end of 
the collar body-cavities, and that at this point the body cavity 
enlarges and begins to hollow out. The ectodermic tube 
pushes into the body-cavity and the somatic wall of mesoderm 
