CILIATION OF EOTODERM OF AMPHIBIAN EMBRYO, 473 
mucous secretion, which, produced by the glands, is driven out 
by the system of water-currents described above. 
Special Currents upon the Dorsal and Lateral 
Surfaces. 
Although I cannot speak with certainty with regard to the 
earliest stages, yet as far as I have been able to determine I 
have never found a stage in which every cell of the epidermic 
epiblast bears cilia. 
In 6 mm. tadpoles ciliated cells are scattered thickly over 
the whole surface, but amongst them are other cells which bear 
no cilia. On certain regions the ciliated cells are more 
numerous, and the cilia on certain spots are longer. 
I described above a very well-defined current, marked N.B.C. 
in fig. 5. There is still in older tadpoles a current which is 
swifter than any other (excepting those connected with the 
glands described above), and very well indicated, which seems 
to be identical with that marked N.B.C. in fig. 5. I have simi- 
larly named it N.B.C. in fig. 12, of which I am now writing. 
This current dips into the nasal depression which is pro- 
vided with long cilia, and flows rapidly over the developing 
external gill-filaments. 
This conspicuous current exists until the posterior nares are 
formed and the gill-filaments are covered over by the oper- 
culum, whereupon the current is no longer distinguishable as 
a special stream, but is merged in the general much-reduced 
flow of water which sweeps slowly from before backwards over 
the whole body. This is an interesting current, for it will be 
noticed that the greater part of the water which washes the ex- 
ternal gill-filaments until the tadpole is about 9 mm. long has 
previously passed over the developing olfactory epithelium. 
Whether this epithelium at this stage plays any part in testing 
the water before it reaches the gills must be left to conjecture. 
Fig. 15 is drawn from a horizontal section of a 6 mm. tad- 
pole. It represents the ectoderm along the path of the current 
just described, and is between the olfactory pit and the base of 
the external gills. The ciliated cells are here very numerous, 
