NORMAL MODE OF SECRETION IN THYIIOID GLAND 45 



mittent function of the thyroid cells; (2) there are other, at pres- 

 ent unknown mechanisms for the formation of colloid, correlated 

 with droplet formation, but able to proceed without it; (3) the 

 secretion of colloid into the gland lumen is an accessor}^ and not 

 the primary function of the epithelial cells of the gland. 



For the reasons mentioned above, it is apparent that the for- 

 mation of colloid droplets in the cell, at least, is an intermittent 

 ■ function. It is possible, however, that in addition to this mode 

 of formation of colloid there is a slow and continuous production 

 of colloid at the free margin of the cell unaccompanied by the 

 formation of visible secretion antecedents in the cytoplasm, and 

 it may be that this latter is the main method of production of 

 intrafoUicular colloid, the droplet method representing some 

 upset of secretory equilibrium which results in the accumulation 

 of the product of secretion in the cell, instead of the lumen. 

 That this hypothetical upset is of the nature of an acceleration 

 of the secretory rate is, however, excluded by the fact that we 

 often see the droplet formation in the greatest abundance in 

 adenomata the stroma of which is in an advanced state of hyaline 

 degeneration, and in which, therefore, there can be no question of 

 accelerated secretion rate. Histologically considered such a con- 

 ception of the process of secretion in the thyroid gland must 

 remain hypothetical, since it is incapable of objective proof. 



The third possibility, namely, that the secretion of colloid in- 

 to the gland lumen is an accessory and not the primary function 

 of the epithelial cells, though correlated intimately with this 

 primary function, would, if established, explain and include all 

 of the facts. Such a theory to be accepted, must account for the 

 irregular occurrence of droplets, for their formation in the inte- 

 rior of the cell rather than on either of the free surfaces, and 

 for their increase under iodin or thyreoglobulin administration, 

 and under the experimental conditions of Hiirthle. It involves 

 the assumption of a more remote antecedent of the secretion than 

 the colloid droplets of Hiirthle. 



In my studies of the thyroid glands of \'arious mammals, I 

 have been struck with the frequent occurrence, particularly in 

 the cat, dog and opposum, of vacuoles with unstainable con- 



