-44- 



pharynx. (^'ag, 325 diam, ) 



Fl^« 3 e^ Transverse section through the gland, poc- 

 terior to the opening Into the pharynx. (Wag, 325 diam.) 



Pl^, 3^, vSagittal section through the head of a IS-day 

 P. planer! larva , passing almost through the median plane of the 

 thyroid and somewhat to one side of the centre of the stomo- 

 daeiim. Shov/s the beginning gland as an evagination of the floor 

 of the pharynx. The anterior and posterior horizontal lamellae, 

 which finally separate the gland from the pharynx, are just be- 

 ginning to be formed. (Kag, 325 diam.) 



(Figs, 4-7 are taken from the material obtain-sJ at 

 Ithaca, V. Y, , and are therefore either of the lake or of the 

 brook lamprey or, perhaps, of both). 



Fig. 4 a. Transverse SBCtion through the anterior end 

 of the thyroid, passing through the extreme posterior edge of the 

 vertical lamella. Shows beginning of differentiation of cells 

 into "gland" and "cover" cells. (Mag. 240 diam.) 



Fig. 4 b. Transverse section posterior to Fig. 4a , 

 cutting the gland just anterior to the duct, and shov/ing the 

 first trace of the ciliated groove. (Mag, 240 diam. ) 



Fig. 4 c. Transverse section near the posterior end 

 of the gland, posterior to the duct and to the lateral invagina- 

 tions. (Mag. 240 diam.) 



Fig. 5 a. Lateral view of the head of a 9mm. larva, 

 to show the relative size of the thyroid gland, and its position 



