-20- 



brane, a number of triangular condensations of the cell sub- 

 stance on each side of the opening. These inverted cones were 

 seen in a number of sections that happened to be stained in 

 just the proper v/ay, but no explanation for their presence sug- 

 gested itself. 



Roughly speaking, each half of the thyroid may be con- 

 sidered to be made up essentially of two large and two small 

 cylinders of gland cells, each cylinder extending the whole 

 length of the gland (Fig. 5 e, gr. and gr'.). The small cylin- 

 ders (gr',) are as yet so rudimentary that they can scarcely be 

 recognised as such, but, as has been said, they will acquire lat- 

 er the same structure as is now possessed by the larger cylin- 

 ders (gr.). Extending the entire length of each cylinder or 

 cell group is a narrow band of minute holes, mentioned above 

 (Pig. 5 f, o. and Fig. 6 1, m, ) , through which the secretion of 

 the gland cells escapes into the ciliated chambers and thence, 

 through the duct, to the ciliated grooves of the pharynx. 



The remaining structural details of the gland may be 

 understood from a description of the duct, which is somewhat com- 

 plicated. As has been described above, the two ventral branch- 

 es of the ciliated grooves run along the floor of the pharynx, 

 getting closer and closer together until they unite and form a 

 single deep groove (Figs. 5 e-h, v.c.g.). In Fig. 5 f the ven- 

 tral ciliated grooves (v.c.g.) have almost united, being sepa- 

 rated only by a wedge-shaped portion of the pharjTigeal floor. 



