-14- 



Their cilia are short and indistiriot. 



The meeoblast (mes.) surrounding the thyroid and pha- 

 rynx Is now quite different from that of earlier stages, being 

 composed of scattered angular cells instead of closely packed, 

 more or less oval cells. The yolk has disappeared. The outline 

 of the pharynx is represented in a somewhat diagrammatic manner, 

 though it was traced as accurately as ijossible under a camera. 



Fig, b shOY/s the wide separation of the dorsal and 

 ventral branches of the ciliated grooves so that the two dorsal 

 branches f'.re now quite near together at the mid-dorsal line 

 (d.c.g.), while the two ventral branches also are seen to approach 

 each other as they pass posteriorly along the walls of the pha- 

 rynx . (v. e.g. ) . The dorsal rrooves continue to approach each 

 other gradually as they are followed posteriorly until they are 

 separated only by a median ridge of ciliated columnar cells 

 (Fig. L d) . At a point a short distance anterior to the thyroid 

 the grooves disappear n.nd the medial ridge continues posteriorly, 

 though diminished in height, as the ciliated dorsal ridge 

 (Fig. 5 k, c.d.r,). This ridge continues to the extreme poste- 

 rior end of the pharynx and enters the oesophagus, of which it 

 forms, for some distance, the ciliated dorsal wall. 



The ventral grooves, as has been said, approach the 

 mid-ventral line of the pharyngeal wall until they unite and lead 

 into the duct of the gland (Fig8.5e-i), Posterior to the 



