The Morphology of Coeloplana. 57 
filament, ending in a closed ring. Through the center of this ring 
the straight central filament issues (Fig. 5). Adjacent cells are 
covered and united by a thin membrane to which the apical filament 
of the cap is joined. (This membrane is found only in C. mitsukurii.) 
The accessory tentacle sheath is lined with masses of cells that 
differentiate into the muscular tissue of the tentacle and the collo- 
blasts surrounding the latter (Fig. 34 and 35). As the growth is 
continuous, all the various stages of development may be observed 
in different parts of the same area of tissue. The details of 
development are somewhat at variance with the processes described 
by Samassa and by SCHNEIDER. 
The undifferentiated cells are arranged in an epithelium of 
hexagonal cells, each of which contains a rather large and sharply 
staining nucleus immeshed in a reticular network. “Nucleoli” are 
also present, scattered about the cell. These nucleoli arrange 
themselves regularly about the nucleus. The nucleoli next draw 
near the nucleus in the center of the cell, leaving a clear open space 
between them and the cellwall. They do not completely surround 
the nucleus but cup over it in a hemisphere. The end of the cell 
opposite the mouth of this cup begins to elongate. At the same 
time the cellwall (or the cytoplasm next it) in this region begins to 
thicken in a spiral ridge. There is a differentiation of the cytoplasm 
at this stage, — that within the area surrounded by the hemisphere 
of nucleoli becoming denser. As the lower part of the cell elongates 
this differentiation is maintained and the denser part of the cyto- 
plasm is drawn out into a cord, surrounded by the clearer cortical 
cytoplasm. The spiral thickening continues to increase as the cell 
elongates until there are a number (four or five) complete spirals 
formed. At about this time, the cell has an elongated pear shape, 
— the lower part is filled with a spiral, enclosing a dense cord of 
cytoplasm that is rapidly becoming filamentous, — the nucleoli 
surround the rather flattened nucleus in the upper end of the cell 
and about each nucleolus the cytoplasm begins to cleave off in seg- 
ments. Later the nucleoli disappear but the segmentation persists 
and results in the peculiar mulberry-like appearance of the cyto- 
plasm of the fully formed cell. After four or five of the spiral turns 
in the lower part of the cell have been completed, the lower margin 
of the cellwall breaks around, — the spiral separating from the 
cellwall and becoming free and the broken edge persisting as the 
skirt or frill. described above, about the lower margin of the cell. 
