THE DUCTLESS GLANDS 419 



Kishi definitely states, as did the older observers, that the 

 parathyroids are embryonic thyroids. The present writer, in 

 conjunction with W. A. Jolly, found that parathyroids left 

 behind after thyroidectomy can under certain conditions 

 develop in the direction of thyroid tissue, and a functional 

 replacement also takes place. 



The two structures are derived from very similar sources, 

 and even in the fully developed state there is no fundamental 

 difference between their constituent cells. The intervesicular 

 tissue of the thyroid is practically identical with parathyroid, 

 and parathyroid has only to contain colloid vesicles in order to 

 constitute itself thyroid. This view has recently been confirmed 

 from the standpoint of comparative anatomy by Forsyth (27), 

 and further evidence will soon be published from the Physio- 

 logical Laboratory at Winnipeg. It is further to be noted 

 that in structures which are topographically parathyroids we 

 sometimes find colloid vesicles, especially in the human subject. 

 It is a matter of no importance to the present discussion 

 whether these are normal or pathological. It is impossible, 

 then, to avoid the conclusion that thyroids and parathyroids 

 are morphologically and functionally very intimately related. 

 Perhaps it is not going too far to say that we should look upon 

 the two structures as forming one apparatus. 



(c) Chemistry of the thyroid. — The question naturally arises, 

 what is the chemical nature of the active principle of the thyroid 

 gland ? What is the substance, or what are the substances, the 

 absence of which causes such serious metabolic disturbances in 

 some animals ? We may say at once that there is at present 

 no satisfactory answer to the question. There have been a large 

 amount of work and much discussion on the subject. Baumann 

 discovered the presence of iodine in the thyroid glands, and 

 prepared a substance which he called " thyroiodin," and which 

 he thought was the active principle of the gland. But the 

 " activity " of the substance in neutralising the ill effects of 

 thyroidectomy is disputed, and there are other objections to 

 Baumann's theory. Iodine is absent from the thyroids of some 

 animals. Baumann himself states that while the thyroid of a 

 dog fed upon Spratt's dog biscuits contains iodine, this element 

 is absent after a meat diet. In the ox, horse, and pig, iodine 

 may be absent or may be present in the merest traces ; further 

 Baumann admits that iodine cannot always be found in the 



