94 PROCEEDINGS OF THE ACADEMY OF [Feb., 



which uo cilia could be made out. This ridge, after extending 

 forward for a very short distance, becomes changed gradually into 

 a shallow, median, ciliated groove that continues without change 

 to the duct of the thyroid. Just anterior to the duct, the ventral 

 ciliated groove splits into two grooves, which separate gradually 

 on the floor of the pharvnx, 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 backward until, at a point 

 anterior to the front end of the thyroid, they unite again and 

 form the median, ciliated dorsal ridge which extends backward to 

 the oesophagus, of which it forms, for a short distance, the dorsal 

 wall. 



In the gland itself the changes begun in previous stages have 

 been carried now much further. The cover cells (figs. 5e-m, d. I. ) 

 (Decklamelle) have shortened until they form a sort of mem- 

 brane, while the gland cells (figs. 5f-m, g.L) have greatly elon- 

 gated and their nuclei now lie near one end. The gland is com- 

 pletely di\dded by the median lamella (figs. 5e-h, m.l.) from the 

 anterior end as far back as the duct. From the duct to the poste- 

 rior end (figs. 5i—k, m.L), the lamella is simply a partition rising 

 from the floor of the gland but not reaching to the roof, its upper 

 edge being swollen for a considerable distance posterior to the duct 

 (figs, oi-k, m.l.'). The lateral invagination is very deep and 

 important in determining the structure of the organ. Near the 

 anterior end (fig. be, i.v. ) it has not changed greatly from the con- 

 ditif)n in which it was at the last stage, but as we pass to sections 

 furtlier back we find that the invagination forms a considerable 

 cavity in each lateral half of the gland (figs. 5f-h, i.v.), and 

 posterior to the duct the gland, in cross section, is seen to be divided 

 into three distinct parts, a central and two lateral divisions (figs. 

 5i-k). In the posterior part of the invagination mesoblast cells 

 are often seen (fig. 5i, mes.). 



The most extensive and complicated changes that have taken 

 place are those of the gland cells (g.l.). The first among the 

 changes to be mentioned is the separation of these cells into two 

 groups, the ciliated cells and gland cells proper. The method of 

 this separation is partly shown in fig. 5e. As seen in this section, 

 the nuclei of the gland cells appear to have arranged themselves 

 near the periphery of a somewhat oval mass of cells and to have 



