-10- 



expanded at the ijottoci where a narrow space is Isft, on either 

 side, betv;-een the thlok lateral and thin ventral walls (Fig, Cd) . 

 Along this croove-like part of the thyroid the cells of the 

 pharynx and gland pass quite insensibly into one another, and 

 are not sharply distinguishable as might be supposed from the 

 stippling in Figs, 2 c and 2 d. The portion of the thyroid pos- 

 terior to the opening is a simple tube-like structure, nearly 

 circular in outline and with a vertical slit-like cavity 

 (Fig, 2 e). The >7alls here are of nearly the same thickness 

 on all sides, and lie in contact with the epithelluin of the 

 pharynx above and with that of the body wall below; jn fact, 

 throughout almost the entire length of the gland this is the case. 

 On each side of the thyroid throughout its entire length lies a 

 pseudobranchial artery (art.) ( "Rpritzlooharterie" of Hohrn) , 

 There is no indication, as yet, of the ciliated grooves, "Pseudo- 

 branohialrinne. " 



The stages of development which follow were obtained 

 from thsr Ithaca, mfiiftria ^., -h icb ms:isi^i^, it will be remember^jd, x 



« 

 we re 



tsa=a possibly of two species neither of which was identical with 

 th e.f rapl e^ Jmat e r laj, , P, planer i. 



The next change that is noticed in the thyroid gland 

 is the development of the median vertical lamella (Fig. 4 a, 

 m.e.) which is formed by the invagination of the thin dorsal 

 and ventral walls, beginning at the anterior end and gradually 

 extending posteriorly until, at this stage, the gland is separated 



