CILIATION OF ECTODERM OF AMPHIBIAN EMBRYO. 469 
The action of the cilia is strongest at the anterior end, and 
causes the water to be driven backwards as from a centre, the 
centre being the most anterior end of the embryo; in fact, just 
as would occur if the embryo was swimming rapidly forward. 
The cilia on the anterior surface can at this stage be seen 
distinctly with a Zeiss D objective. There are, however, 
certain tracks along which special currents of water flow, 
which we may take to mean that along these paths the cilia 
are especially large, more numerous, or more active. 
A very well-defined and strong current (fig. 5, N.B.C.) passes 
over the bases of the developing branchial arches, which would 
seem to correspond to that marked by the large arrows in fig. 4. 
Along the ventral surface the motion is extremely slack—it is 
rather a series of eddies. The currents at the hinder end are 
interesting. The action of the cilia of the whole of the hinder 
region is to tend to cause a current of water to flow towards 
the blastopore and anus. The most rapid current is that along 
the back. 
At one time—when we may presume both blastopore and 
anus are open—there is a strong exhalent current from the 
positions of both of the openings. The water brought by the 
action of the cilia may be seen to curl over the edges of the 
blastopore and anus, and apparently is immediately shot out 
again with considerable violence (fig. 6). 
Whether this indicates any interchange of fluid between that 
within the archenteron and the medium in which the embryo 
lies I cannot say. The anal current becomes stronger while 
the blastoporic becomes rapidly weaker, and soon ceases with 
the final closure of the blastopore. 
Fig. 7 illustrates the direction of the currents of water over 
the posterior end a few hours later, at which time the tail has 
begun to grow out. 
Special Currents along the Ventral Surface. 
Until about now the ventral and antero-ventral currents 
have been quite simple; there is a general flow along the 
