30 MORGAN. [Vou. IX. 
pseudopodial-like projections on the inner side to the wall of 
the body-cavity and to one another by similar processes. In 
every respect these cells resemble the mesenchyme cells of the 
blastocoel and show no trace of origin from either the meso- 
derm of the body-cavity on one side or from the ectoderm on 
the other. The cells are, I think, the beginning of the pro- 
boscis vesicle. 
A much later stage in the history of the proboscis vesicle is 
shown in Figs. 11 and 12. The more anterior section (Fig. 11) 
is near the junction of the body-cavity and its short exit tube. 
The proboscis vesicle (/.v.) is more or less spherical in cross 
section. At one point two large cells are found running from 
the body wall to the proboscis vesicle, and another cell clearly 
inside the ectoderm closely resembles these two; whether or 
not this has anything to do with a proliferation of ectoderm at 
this spot I am unable to say. In Fig. 12 the section is nearer 
the periphery of the sphere. Here, one cell that forms part 
of the wall of the vesicle is clearly attached to the ectoderm 
as well. 
In my previous account I have described what I believed to 
be a mesodermal (mesenchyme) origin of the proboscis vesicle, 
and I see as yet no reason to change this view. It must be 
admitted that it is difficult to see why so close a connection 
seems to be present between the ectoderm and the cells of the 
vesicle. On the other hand, all the direct evidence points un- 
mistakably to a mesenchymetous origin. As before, I must 
take issue with Spengel on this point. It is barely possible 
that we are dealing with an organ having a double origin, vzz., 
from mesenchyme and ectoderm ; but there is little evidence 
for such an assumption. 
Again, at this stage, the third pair of body-cavities arises, 
and a little later the second-paired body-cavities arise. We 
may study the third pair first. Returning to Fig. 6, we find 
just inside of the circular ciliated band, a mass of tissue, with 
conspicuous nuclei and faintly stained protoplasm. This is an 
early stage of the last or third pair of body cavities. At this 
stage the tissue extends around the right and left sides, but does 
not meet in the mid-dorsal and mid-ventral lines. It touches 
