oV 



anterior end of ths^-^Pe tr omy zon^hyroidj (Fig. lb.) shows it 

 to be a closed cavity ■while posteriorly it is still an open 

 groove (Pig. 1 o.). In Amblystoma, on the other hand, the open 

 groove is anterior to the closed part of the gland, the closing 

 in taking place apparently in a postero-anterior direction. 



(13th and 14th days.) At about this time the lateral 

 walls of the thyroid, especially towards the anterior end, 

 become greatly thickened by the increase In length of the co- 

 lumnar cells of which they are composed. At the same time an 

 invagination of these thickened walls occurs as a longitudinal 

 groove pushing inv/ard and upward from the outside of the gland 

 (Fig. 2b,l.v. ) . This invagination becomes still more larked 

 In later stages and extends farther in a posterior direction. 



At this stage the extreme anterior end of the thyroid 

 is forked, so that a section through this region would show 

 two distinct cavities with thickened lateral and ventral walls 

 (Fig, 2 a). This is plainly seen in sections and in a wax re- 

 construction of the thyroid made from a series of sections of 

 this stage. These two divisions of the anterior end of the gland 

 are separated by a vortical partition of mesoblast. The two 

 parts of the thyroid are more widely separated by the mesoblast 

 than they will be p.t later stages, but at this stage the sepa- 

 ration extends only a short distance from the anterior end of • 

 the gland. The thyroid is here not so closely pressed against 



