CH. XV] ORGANS FROM THE ECTODERM 167 



The gill-filaments also carry cilia in the proportion of one 

 ciliated cell to two non-ciliated cells. The effect of the cilia 

 becomes less conspicuous after the larvae have reached 7 or 8 

 mm. in length, although even in much later stages the cilia are 

 still found over all parts of the body. Their motion is suffi- 

 ciently strong to cause the embryo (6 or 7 mm. in length) to 

 move forward, if placed on a glass plate, at the rate of one 

 millimetre in from four to seven seconds. 



