2 MORGAN. Vox. IX. 
3 
It will be seen that these groups arise independently of one 
another, and are separated by quite wide spaces of the blasto- 
coel cavity. Moreover, the cells are at first not in quite such 
close proximity to the inner wall of the ectoderm as in the 
later stage already studied (Fig. 6). From this early stage to 
the stage already studied and drawn in Fig. 6, all intermediate 
stages are found. We can therefore affirm that the third pair 
of body-cavities arises from cells of the blastocoel space, far 
removed from the endoderm ; that this pair of body-cavities does 
not have a single beginning, but comes from several groups of 
cells that ultimately fuse into a pair of body-cavities lying at 
the sides of the larva. 
It is more difficult to trace the origin of the second pair of 
body-cavities (collar-cavities). This is owing to the fact that 
there is present no landmark for finding the exact point where 
they appear, and also in a greater degree to the frequent col- 
lapse of the wall where they originate, rendering a determina- 
tion difficult, as tangential sections are often found. 
Even under these conditions the answer is definite. The 
body-cavities arise from mesenchyme cells, lying just beneath 
the ectoderm, and although I cannot state with the same posi- 
tiveness as in the former case, yet there seems to be no doubt 
that the early beginnings appear zot to be single for each side, 
but to come from the fusion of several groups of cells. 
In Fig. 6, a later stage of the second body-cavity is found at 
b.c.2. At this stage the body-cavity is a continuous mass on 
each side. 
As Stage 3 represents the highest point of development 
reached in the pelagic life of the larva, we may study it some- 
what more fully. 
Small portions of the circum-oral or extra-oral area are 
shown in Figs. 16 and 17. In the first of these from the 
circum-oral area, the number of nuclei is shown, and also the 
presence of deeply-stained (in Haematoxyln) gland cells. In 
the surface view of the extra-oral area (Fig. 17), most of the 
nuclei are large and indistinct, but amongst these are smaller 
and deep-stained nuclei. Along the course of the ciliated band 
