-16- 



after extending forwards for a very short distance, becomes 

 changed cjradually into a shallow, median, ciliated groove that 

 continues without change to the duct of the thyroid. Just ante- 

 rior to the duct the ventral ciliated groove splits into two 

 grooves, which separate r.radually, on the floor of the pharynx,, 

 and, near the extreme anterior end, pass upward on each side 

 to the roof of the pharyngeal cavity. The grooves then converge 

 as they pass backwards until, at a point anterior to the front 

 end of the thyroid, tbey unite again and from the median, cili- 

 ated, dorsal ridge which extends backwards to the oespohagus, 

 of which it forms, for a short distance, the dorsal wall. 



In the rrland itself the changes begun in previous 

 stages have been carried now much further. The cover cells 

 (Figs. 5 e-m, d.l,) (necklarnelle) have shortened until they nov.' 

 form a sort of mem>:rane, while the gland cells C^igs. G f-m, 

 g.l.) have greatly elongated and their nuclei now lie near one 

 end. The gland is completely divided by the median lamella 

 (Figs, 5 e-h, m.l.) from the anterior end as far back as the 

 duct. ^rom the duct to the posterior end (Figs, b x-k, m.l,) 

 the lamella is simply a partition rising from the floor of the 

 gland but not reacMnr; to the roof, its upper edge beinp swollen 

 for a considerable distance posterior to the duct (Figs^ i-k, 

 m.l,). The lateral invagination is very deep and important in 

 determining the structure of the organ. Tear the anterior end 

 (Fig. 5 e, i.v.) it has not changed greatly from the condition 



