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Scientific Proceedings (132) 



membrane into the follicle, giving rise there to the marginal zone 

 of red colloid around the yellowish brown, green, or blue center 

 of the colloid mass. 



Only one kind of stainable colloid is formed, the red colloid. 

 It stains yellow, green, and finally blue, when it becomes old. 

 It undergoes this change no matter if it ages in the follicle 

 or within the cell. Blue colloid is not indicative of an actively 

 releasing state of the thyroid, but of an accumulation of old col- 

 loid. Therefore, the colloid in the thyroid of old axolotls, which 

 elaborate but do not release colloid, stains deep blue, almost 

 black. 



No explanation has been found as to how the colloid escapes 

 from the follicle in the blood. It appears, however, that the 

 vacuoles are somehow connected with the colloid release. There 

 are two kinds of vacuoles in the colloid : closed vacuoles, and 

 those which are in open communication with the inner cell ends. 

 When the colloid first appears, it has no vacuoles. The colloid 

 of all larvae prevented from metamorphosing, by experimental 

 procedures, and the colloid of larval axolotls are practically de- 

 void of vacuoles. Apparently, the elaboration of stained colloid 

 may take place in the absence of vacuoles. Yet, in the colloid 

 of normal larvae, the vacuoles increase steadily in number and 

 size. Not only within the follicle, but also within the cell, the 

 colloid develops vacuoles upon aging. 



Communicating vacuoles are found only in thyroids releasing 

 the colloid into the blood. They appear suddenly, when meta- 

 morphosis begins. At the same time, large vacuole-like spaces 

 appear in the cells causing the swelling of the cells. It seems as 

 if the content of the vacuoles is poured into the cells and es- 

 capes through the periphery into the blood. At any rate, com- 

 municating vacuoles are indicative of colloid release. 



Feeding inorganic iodine to normal larvae and axolotls, al- 

 though it does not enforce the release of the colloid, produces 

 ordinary vacuoles. 



The elaboration of colloid is not necessarily followed by the 

 release of the colloid. Feeding of inorganic iodine to old axo- 

 lotl larvae results in an increased rate of colloid elaboration, pro- 

 ducing a wide marginal zone of red colloid; yet metamorphosis 

 cannot be enforced by this procedure. Larvae fed exclusively 

 thymus, do elaborate colloid; yet they cannot metamorphose. 

 Colloid elaboration and colloid release are, within certain limits, 

 independent of each other. 



