1902.] NATURAL SCIENCES OF PHILADELPHIA. 97 



have almost united, being separated only by a wedge-shaped por- 

 tion of tlie pharyngeal floor. They are deep, aud so narrow that 

 there would seem to be but little room for ciliary action. The cells 

 lining the grooves are here somewhat shorter than the other cells of 

 the pharynx, whereas more anterior sections showed the reverse to 

 be the case, as was mentioned above (fig. ob-d). 



The single ciliated chamber (considering one side, only, of the 

 bilaterally symmetrical gland) forms about three-fourths of a circle 

 {m.c.h.) and is bounded by the cover cells {d.l.) on the one hand, 

 and by the ciliated cells (/./.) on the other. The cover cells have 

 become so much flattened that they form a mere membrane, and 

 their nuclei are pressed far apart. The ciliated cells are of a typical 

 ciliated columnar form, and are exposed, at their ciliated ends, to 

 the cavity that has been described as the ciliated chamber, and, at 

 their basal ends, to the space that is now a part of the lateral uivag- 

 ination (i.v. ). Several large blood vessels are seen, surrounded 

 by the angular mesoblast cells, and on each side of the thyroid, 

 lying close to the body wall, is a large longitudinal muscle. Before 

 reaching the point at which it opens into the thyroid, the groove 

 spreads out at the bottom (fig. bg, p.) and resembles, in cross 

 section, an inverted letter T. The cross arms of the T turn upward 

 as they pass further backward (fig. 5 A, p. ) until they are closely 

 pressed against the vertical part of the T. The way in which this 

 curious groove opens finally into the thyroid is interesting. A 

 short distance back of the point repi'esented in fig. 5/t, the cells of 

 the ciliated and cover layers are interrupted at about the point 

 marked b.r. (fig. 5A), and at the same time the side pouch (p.) 

 of the groove (v. e.g.) becomes separated from the groove itself 

 along the line x-y (fig. 5h). By the union now of the medial 

 end (1.) of the ciliated layer with the end (1'.) of the adjacent 

 side of the groove, and at the same time by the union of ends 2 

 with 2' and 3 with 3', we have a condition represented in fig. 5i. 

 The end (4) of the cover layer unites with its fellow of the opposite 

 side of the gland to form the swollen upper edge of the partition 

 (m.l., fig. 5t) mentioned above. By a careful comparison of figs. 

 dh and 5i, the relationships just described will become clear. 



In fig. 5i then we have represented a section passing directly 

 through the duct of the gland, and we see that the secretion from 

 the two groups of gland cells (g.r.) passes out of the gland in two 



