648 DAVID HILT TENNENT. 
Throughout its extent the lumen of the tube contains cer- 
cariz in various stages of development. 
Sections (figs. 7, 8, 17,) show that the tubes have a thin 
homogeneous cuticle beneath which lies a layer of circular 
and a layer of longitudinal muscle-fibres. In many cases 
sharp constrictions appear in the tubes; these, I believe, are 
caused by contraction of the circular muscles. I have never 
been able to see any indication of movement, other than this, 
in the sporocyst. 
The wall of the tube in its younger portions (figs. 7 and 17) 
is about 0:002 mm. thick, the thickness varying greatly with 
the age of the tube. In this younger portion the limits of 
the cells which compose the wall are fairly well defined. 
These cells have a uniform appearance, the finely granular 
cytoplasm staining much more lightly than the vesicular 
nuclei with their scattered nuclear material (fig. 7). 
In slightly older tubes the original definite cell boundaries 
have disappeared and the differentiation of germ-cells has 
commenced (figs. 6 and 17). Within the lumen of the tubes 
undivided germ-cells and germ-balls may be seen. 
In the oldest tubes (fig. 8) the walls have decreased in 
thickness, cell boundaries have disappeared completely, the 
muscles may no longer be recognised, the differentiation of 
germ-cells and their subsequent endogenous proliferation has 
ceased, and only a few scattered nuclei remain. 
The germ-tubes in their earlier growth push their way in 
the intercellular spaces in the tissues of the host (fig. 14). 
Later, an actual destruction of these tissues seems to take 
place, a definite zone of apparently disintegrated cells (Pale- 
tot ?) lying between the wall of the tube and the unaffected 
cells (fig. 6). In whatever manner it is accomplished the 
cells composing the tissues of the host ultimately disappear 
in great part, only a few shrunken strands remaining. In 
the reproductive organs the formation of the sexual products 
is practically completely inhibited. While a few eggs or 
sperms may be present their appearance is abnormal. 
At the beginning of this section I spoke of the youngest 
