38 MORGAN. [Vou. 1X. 
one point in the antero-ventral, where the walls of the two 
cavities are in contact, a proliferation is found. Such a figure 
does not show the right prolongation of the body-cavity that 
lies on the side of the proboscis vesicle towards the observer. 
With this, by way of preface, we may examine a series of 
sections of the structures in the places indicated by the parallel 
lines of Fig. 32: 
In Fig. 33 A the anterior end of the proboscis vesicle is cut ; 
in B, nearly the same conditions are found ; in C, we find in the 
upper part of the section a space, and in J this space opens 
into the blastocoel space; in /, we find on each side of the 
proboscis vesicle the prolongation of the body-cavity, and the 
proboscis vesicle is exposed on its dorsal and ventral surfaces to 
the blastocoel space. In this same section the internal open- 
ing of the exit-tube is found on the left (right in figure) pro- 
longation. Lastly, in /, the external opening of the exit-tube 
is seen, and this is the last section cutting through the cavity 
of the proboscis vesicle. 
That portion of the wall of the proboscis vesicle that is 
exposed to the blastocoel has its cells elongated from side to 
side. These fibre-like cells have highly refracting borders, 
and the nucleus in the middle. Each cell only extends over 
a short portion of the wall, and has pointed ends. A portion 
of the wall is drawn in Fig. 34. The nuclei lie on the inner 
side of the cells, while the walls of the cells, represented by 
lines in the figure, are highly refracting. These cells have 
clearly differentiated into muscle-cells, and, in the living 
animal, during all the later stages, the proboscis-sac beats 
rythmically. 
It has been seen that, over the area where the anterior wall 
of the proboscis vesicle is applied to the wall of the body- 
cavity, the walls of the body-cavity project into the interior of 
the organ itself (Fig. 32d). The cavity of the blastocoel is 
prolonged at the anterior end of the proboscis-cavity into the 
space bounded by the inturned wall of the body-cavity, and 
this space is not filled with the same jelly-like fluid of the 
blastocoel, but with a more watery fluid. Fig. 32 4 is drawn 
to show this condition. It is taken from a series cut longi- 
