Ch. XV] 



ORGANS FROM THE ECTODERM 



165 



The Appearance of Cilia on the Surface of the 



Embryo 



If the living embryo be examined at the time when the 

 neural folds have appeared, it will be seen that the embryo 

 slowly rotates within the jelly-capsule. This rotation is the 

 result of the activity of certain ciliated ectodermal cells. The 

 distribution of these cells over the surface of the body has been 

 recently described by Assheton ('96). Assheton states that 

 at the time when the medullary folds are first visible, and even 

 after they have begun to roll in, there are no traces of cilia on 



Fig. 51. — Embryo of Rana. The arrows show the direction of currents of water 

 over the surface. A. Side view. B. Ventral view. (After Assheton.) 



the surface of the embryo. Before the neural folds have met 

 in the middle line the ectoderm has become ciliated in certain 

 regions, as can be demonstrated by the streaming movement 

 of granules of carmine placed on the surface. The arrows in 

 Fig. 51 show the direction of the flow of granules over the sur- 

 face. The lateral edges of the anterior end of the medullary 

 folds seem to show the first traces of cilia, and a few hours 

 later (Fig. 51, A) cilia have also appeared along the sides of 

 the folds. 



