1902.] NATURAL SCIENCES OF PHILADELPHIA. Ill 



Fig. 5i. — Transverse section through the opening of the duct into the 

 ciliated groove. Shows that the median ciliated chamber opens directly 

 into the deep ciliated groove, while the lateral ciliated chambers open 

 into the ciliated groove tlirough the side pouches noticed in the last two 

 preceding figures. The lateral invaginations, from this point to the pos- 

 terior end, divide the gland into three distinct parts, a median and two 

 lateral parts. The median lamella is no longer a complete partition and 

 shows a decided swelling on its dorsal edge, (Mag. 240 diam.) 



Fig. 5;'. — Transverse section a short distance posterior to the opening 

 of the duct. The ciliated groove is still present but much diminished in 

 depth. (Mag, 240 diam.) 



Fig. 5^. — Transverse section posterior to fig. 5j, passing through the 

 beginning posterior coil. Shows the now broad and shallow ciliated 

 groove close to the top of the gland, and ciliated dorsal ridge on the roof 

 of the pharynx. (Mag. 240 diam. ) 



Fig 51. — Transverse section through the extreme posterior end of the 

 gland. (Mag. 240 diam.) 



Fig. 5m. — A horizontal section through the thyroid, showing the 

 extent of the complete median lamella, the relative position of the vis- 

 ceral arches, etc. (Mag. 240 diam.) 



Fig. 6a. — Transverse section through the anterior end of the thyroid 

 of a 10 cm. larval Petromyzon. All four groups of cells are now fully 

 developed, and the gland has increased greatly in size, as is shown by 

 the fact that figs. 5a-m were magnified 240 diameters, while the larger 

 figs. 6a-i were magnified only 90 times. This figure about corresponds 

 to Fig. 5e of the precedmg stage. 



Fig. 6b. — Transverse section posterior to fig. 6a and just anterior to the 

 opening of the duct. Corresponds to fig. 57i of the preceding stage. 

 (Mag. 90 diam.) 



Fig. 6c. — Transverse section through the duct of the gland. Corre- 

 sponds to fig. 5i of the preceding stage. The lateral ciliated chambers 

 are much more extensive than in the preceding stage. (Mag. 90 diam.) 



Fig. M. — Transverse section just posterior to the duct. Corresponds 

 to fig. 5j of the preceding stage. (Mag. 90 diam.) 



Figs. Ge-h. — Transverse sections through the coiled posterior end of the 

 gland, cut in the planes indicated in fig. Q/c. (All mag. 90 diam.) 



Fig. 6i.— Transverse section through the thyroid, posterior to the coiled 

 posterior end of the gland, to show tlie manner in which the lateral pairs 

 of cell-groups extend for some distance behind the coiled median cell- 

 groups. The pseudobranchial arteries are seen in this section as a single 

 large vessel. (Mag. 90 diam.) 



Fig. Gj. — Ventral view of the head of a 15 cm. larva, with the ventral 

 integument drawn to each side to expose the thyroid gland. The relation 

 of the gland to the bars of the brancliial basket is shown. (Mag. 3 

 diam.) 



Fig. Qk. — Diagrammatic lateral view of the thyroid gland, recon- 

 structed by measurement to show, more or less accurateley, the lonrji- 

 tudinal relationships of the various parts of the gland. The doited lines 

 indicate the planes through which tlie sections (fig^- 'on-i) pass. 



Fig. Ql. — Section through the opening of one of the cell-groups, 

 highly magnified to show details in structure. (Mag. 925 diam.) 



Fig. 7. — Larva of a stage between those represented in figs. 5 and 6. 

 About 2 J cm. in length. 



Fig. 8a.— Transverse section through the pharynx and anterior end of 

 the thyroid in Amblystoma, showing the latter as a deep, ciliated groove 

 in the floor of the former. (Mag. 240 diam.) 



Fig. 86. — Transverse section posterior to fig. 8a, showing the thyroid 

 as an enclosed tube lined witli cilia. (Mag. 240 diam.) 



