THE FOREGUT 



201 



ing cartilaginous elements expand, the major blood vessels become 

 enmeshed in them, and eventually they come to lie close to the heart. 

 At the forelimb emergence stage of metamorphosis there is a marked 

 distension of the follicles, and by the tail resorption stage there is high 



34^*'i*S?'*"" 







-vjl 





Follicular 

 epithelium 



Follicular 

 colloid 





Partially vacuolated 

 colloid 



Early development of the thyroid gland of the frog — {Continued) . (E) Earli- 

 est differentiation of the thyroid follicles. (F) Thyroid gland of the hindlimb 

 bud stage. (G) Same as "F" but entire gland at low power. (H) Thyroid gland 

 of the forelimb emergence stage. (I, J) Active follicular secretion of the thyroid 

 of the metamorphosing frog. (K) Thyroid gland of the frog immediately after 

 metamorphosis. 



