-48- 



Flg. 6 o. Transverse seotlon through the duct of the 

 gland. Corresponds to Fig. 5 j of the preceding stage. The lat- 

 eral ciliated chambers are much more extensive than in the pre- 

 ceding stage. (Mag. 90 diam. ) 



Fig. 6 d. Transverse section just posterior to the 

 duct. Corresponds to Fig. 5 j of the preceding stage. {K&g, 

 90 diam, ) 



Figs. 6 e-h. Transverse sections through the coiled 

 posterior end of the gland, cut in the planes indicated in 

 Fig. 6 k. (All mag. 90 diam.) 



Fig. 6 i. Transverse section through the thyroid, 

 posterior to the coiled posterior end of the gland, to shov/ the 

 manner in which the lateral pairs of cell -groups extend for some 

 distance behind the coiled median cell-groups. The pseudobranch- 

 ial arteries are seen in this section as a single large vessel. 

 (Iv^ag. 90 diam. ) 



Fig. 6 j. Ventral view of the head of a 15cm. Petro- 

 myzon Li s WA fS, with the ventral Integument drawn to one side to 

 expose the thyroid gland. The relation of the gland to the bars 

 of the branchial basket is shov/n. (Mag, 3 diam.) 



Fig, 6 k. Diagramraatic lateral view of the thyroid 

 gland, reconstructed by measurement to show, more or less accu- 

 rately, the longitudinal relationships of the various parts of 

 the gland. The dotted lines indicate the planes through which 

 the sections (Figs. 6 a-i) pass. 



