468 
HENRY F. NACHTRIEB 
rounding a stained and granular inner portion. The free end of 
the flagellated cell is a short cone in which the cytoplasm is homo- 
geneous and has a faint bluish tint. The apex of the cone is 
extended as a slender deeply stained flagellum through the open- 
ing between the outer ends of the contiguous cover cells. At the 
base of the flagellum there are two distinct deeply stained basal 
bodies (diplosome), one below, the other and apparently in con- 
tact with each other. When I first saw this flagellum I inter- 
preted it as a sensory hair, but the two basal bodies militated 
against this interpretation. Moreover it was found curved in 
various directions, which conditions would not obtain if it were 
a stiff hair. While T have not had an opportunity of verifying 
this interpretation with observations on living material I do not 
hesitate in declaring the process a flagellum, because all the mor- 
phological features are in accord with it. Having been con- 
vinced of the presence of the flagellum in good iron hematox.ylin 
preparations, one now and then gets suggestions of it in sections 
stained with Ehrlich-Biondi and other stains; but none of the 
various stains used, excepting the iron hematoxylin, differentiated 
the flagellum clearly. 
A comparison of fig. 12, representing Kistler's two kinds of 
cells, with my figs. 10 and 11 and the descriptions given above 
will convince one that there are irreconcilable differences between 
the two accounts. T can explain these differences only on the 
assumption that Kistler based his conclusions entirely upon 
observations made with objectives of too low power. 
In the character of the nucleus and the general appearance of 
the cytoplasm the cover cell is like the cells of the middle and basal 
layers of the skin epithelium. The fiagellated cell, on the other 
hand, differs from all of them in every respect. Neither kind of 
cell has the characters of a distinct sensory cell. 
The final conclusion accordingly is: Primarily the primitive 
pore of Polyodon spathula is an excretory organ that throws off 
a peculiar mucus, and the differentiated epithelium of the pore 
consists of a single layer of two kinds of cells on the floor of the 
pit — the cover or mucus cell and the flagellated cell. 
