166 DEVELOPMENT OF THE FROG'S EGG [Cn. XV 



As the medullary folds grow nearer together, the ciliatioii ex- 

 tends further back along the sides of the dorsal surface. When 

 the folds have just touched at the anterior end, cilia appear 

 on the antero-ventral surface of the embryo, in the region where 

 the mouth subsequently forms. The direction of the currents 

 set up is from before backward. The whole of the dorsal 

 surface next becomes ciliated. The ciliatioii spreads rapidly 

 and at the time when seven or eight mesoblastic somites are 

 present (when the embryo is 3 mm. in length) it has extended 

 over the whole surface of the embryo. The currents, however, 

 differ very much in intensity. Figure 51, A, B, shows the 

 direction of the flow, the larger arrows indicating stronger cur- 

 rents. The action of the cilia is strongest at the anterior end 

 of the body. A well-defined stream passes over the bases of 

 the gills, which have begun to appear at this time. Over the 

 ventral surface the currents move slowly and in eddies. At 

 the hinder end of the embryo the action of the cilia directs the 

 currents of water toward the blastopore and anus. When the 

 embryo measures 4 mm., the so-called "suckers" have appeared, 

 and the currents in that region have changed their direction. 

 These " suckers " are in reality mucous glands that secrete a 

 sticky substance by means of which the embryo can fix itself 

 to objects with which it comes in contact. The edges of the 

 glands have well-developed cilia, which direct a stream of 

 water over the stomodseal depression, and thence backward 

 between the glands (Fig. 51, B). In older embryos, when 

 the glands have united to each other in the mid- ventral line, 

 the direction of the currents in this region is altered. The 

 central stream now turns outward around the anterior sides 

 of the glands and passes backward along the sides. In older 

 larvae (8 mm.) small special currents run over the edges of 

 the adhesive glands and into the depressions within the 

 glands. 



The cilia that cause the flow of water over the surface 

 of the embryo are not developed by all the ectodermal cells. 

 Even where the currents are most active, the cilia-bearing 

 cells are slightly less abundant than the non-ciliated cells. 

 Each ciliated cell bears on its outer surface numerous short 

 cilia. 



