652 . DAVID HILT TENNENT. 
cells divides, giving a three-cell stage in which the two 
smaller cells lie across the larger (fig. 15, d). In the five- 
cell stage (third division) the smaller cells have again 
divided, and lie somewhat irregularly about the large cell 
(fig. 15, f). 
In the seven-cell stage (fig. 15, g) two of the smaller cells 
(found in the third division) have again divided, and two of 
these cells have taken a position on the surface of the irre- 
gular clump of cells. These two cells flatten, the flattened 
edges growing out and forming a membrane which encloses 
the young germ ball. These flattened cells apparently divide 
again at least once, since in a series of sections it is possible 
to find four nuclei lying at various points in the membrane. 
These nuclei ultimately disappear, leaving the thin membrane 
which persists during the entire development of the cercaria. 
I have never observed any indication of the throwing off of 
this membrane. It seems to form the basement membrane 
lying below the cuticle in the mature cercaria. 
Up to this time (seven-cell stage) cell outlines in the 
' developing germ-ball are fairly distinct. The amount of 
cytoplasm as compared to the size of the nucleus is small, 
but still a thin layer may be seen about each nucleus. In 
further stages of the developing germ-ball I have been unable 
to observe cell outlines, this structure appearing in the nature 
of a syncytium with an ever increasing number of nuclei. 
This spherical mass of slightly separated nuclei, the germ- 
ball, reaches a diameter of about 0:02 mm., when it begins to 
elongate. Meantime a group of nuclei has appeared toward 
one side of the germ-ball from which has developed the 
pharynx, which does not, however, reach its permanent form 
until considerably later. A group of nuclei in the region 
just above the pharynx gives rise to the gut. This group at 
first forms a rather compact mass, which subsequently obtains 
a lumen (fig. 15, g#.). 
After reaching this stage the germ-ball, as already noted, 
begins to elongate. From one end, which we are now able 
to recognise as the posterior end, two processes, the rudi- 
