110 PROCEEDINGS OF THE ACADEMY OF [Feb., 



Fig. 3.— Sagittal section through the liead of a 13-clay P. planeri, 

 passing almost through the median plane of the thyroid and somewhat 

 to one side of tlie centre of the stomodtBum. Shows the beginning gland 

 as an evagination of the floor of the pharynx. The anterior and poste- 

 rior horizontal hvmellaj, which finally separate the gland from the 

 pharynx, are just beginning to be formed. (Mag. 325 diam.) 



(Figs. 4-7 are taken from the material obtained at Ithaca, N. Y., and 

 are therefore either of the lake or of the brook lamprey, or perhaps of 

 both.) 



Fig. 4a. — Transverse section through the anterior end of the thyroid, 

 passmg through the extreme posterior edge -of the vertical lamella. 

 Shows beginning of difterentiation of cells into "gland" and "cover" 

 cells. (Mag. 240 diaui.) 



Fig. 4b. — Transverse section posterior to fig. 4a, cutting the gland just 

 anterior to the duct, and showing the first trace of the ciliated groove. 

 (Mag. 240 diam.) 



Fig. 4c. — Transverse section near the posterior end of the gland, pos- 

 terior to the duct and to the lateral invaginations. (Mag. 240 diam.) 



Fig. 5a. — Lateral view of the head of a 9 mm. larva, to show tbe 

 relative size of the thyroid gland and its position in relation to the gill- 

 arches, etc. (Drawn'from the living animal under a magnification of 50 

 diam.) 



Fig. 5b. — Transverse section through the head of a larva of Petromy- 

 zon, anterior to the thyroid, through tlie point where (on the right side) 

 the ciliated groove turns from the dorsal toward the ventral side of the 

 pharynx. The section cuts exactlj^ througli the end of the loop on the 

 right side, but somewhat posterior to this point on the left side. (Mag. 

 240 diam.) 



Fig. 5c.— Transverse section a short distance posterior to fig. 56 to 

 show how the dorsal branches of the ciliated grooves approach each 

 other, on the roof of the pharynx, as they pass toward the posterior ; and 

 the same for the ventral branches on the floor of the pharynx. (Mag. 

 240 diam. ) 



Fig. Of/. — Transverse section posterior to fig. 5t" and just anterior to the 

 gland. Shows the dorsal ciliated grooves separated only by a ridge of 

 cells, which, a short distance further toward the posterior, becomes the 

 ciliated dorsal ridge. The ventral grooves also are seen nearer together. 

 (Mag. 240 diam.) 



Fig. 5e. — Transverse section through the anterior end of the thyroid. 

 Shows the still nearer approach of the two ventral grooves. The cover 

 cells are flattened out into a thin membrane, while the gland cells are 

 very long, are splitting off from their nucleated ends the cells to form 

 the ciliated layer and begin to show an arrangement into the four cell- 

 groups. The lateral invagination deeply indents each of the halves into 

 wliicli the gland is divided by the median lamella. (Mag. 240 diam.) 



Fig. 5/. — Transverse section posterior to fig. 5e. The ciliated grooves 

 are nearly joined, the ciliated layer of cells is now distinct, as are two of 

 the four cell-groups. The lateral invagination is very deep. (Mag. 240 

 diam.) 



Fig. 5g. — Transverse section posterior to fig. 5/ but still anterior to the 

 duct. The ciliated grooves have united to form a single deep groove 

 which is here spread out at the bottom, giving it the shape, in cross 

 section, of an inverted letter T. The structure of the gland is now about 

 the same as in the preceding section. (Mag. 240 diam.) 



Fig. 5h. — Transverse section posterior to fig. 5*7 and just anterior to 

 the opening of the duct. The structure of the gland is the same as in 

 fig. 5g, but the shape of the ciliated groove has changed somewhat. 

 (Mag. 240 diam.) 



