1902.] NATURAIi SCIENCES OF PHILADELPHIA. 103 



the very deep ciliated groove shown in this and the next figure. 

 The arteries (art. ) are very large and are bound closely to the 

 gland by the connective tissue mentioned above. The cover layer 

 {d.l.) iu these sections is reduced to such a thin membrane that 

 it is often difficult to distinguish it from the connective tissue to 

 which it is closely applied thi'ougliout. 



A section passing directly through the opening of the duct into 

 the gland is represented in fig. 6c. The duct, a deep, narrow slit, 

 is lined with short, ciliated columnar cells, similar to the shorter 

 cells of the ciliated layer spoken of above. These short cells lead 

 quite suddenly into the long cells of the ciliated layer (f.L). 



The lateral pair of cell-groups (Z.c.gf.) is now entirely distinct 

 from the median pair (m.c.g.), and the pocket (p.) by which the 

 lateral cell-groups are connected with the duct, as before explained, 

 is continued to the extreme posterior end of the gland, the two 

 lateral pairs of cell -groups taking no part in the posterior coil but 

 continuing back of this for some distance (figs. %i and k). 



The median lamella {m.L), which back to this point has com- 

 pletely divided the gland into two parts, is from this point merely 

 a tall ridge (fig. 6c, m.l.) projecting upward into the median cili- 

 ated chamber (m.c.h. ) and covered by the cover cells (d.l. ) (Deck- 

 lamelle) of the two median pairs of cell-groups. As in the pre- 

 ceding stage, the dorsal edge of this ridge is at times somewhat 

 thickened. 



The space between the anterior end of the coil and the duct is 

 short, as is seen by fig. Qk, and a section through the gland in this 

 region (fig. Qd) differs very little from a corresponding section on 

 the preceding stage (fig. 5/), except iu the particulars already 

 pointed out for the anterior end of the gland. The median ciliated 

 chamber (m.c.h.) is long and naiTow in a doreo- ventral direction 

 and its dorsal part is made up of the shorter form of ciliated cells. 

 The latei-al ciliated chamber (l.c.h.) is also much more extensive 

 than in the preceding stage. The ciliated groove (v.c.g.) is seen 

 above the gland in the floor of the pharynx. It is much less deep 

 than it was, just anterior to the duct, and remains in about this 

 condition to a point beyond the extreme posterior end of the gland. 

 There is still a considerable space between the thyroid and the floor 

 of the pharynx, which space is filled with the reticulated tissue 

 mentioned above and shown in figs, (m and 66. 



