194 JEREMIAH S. FERGUSON 



large intrafollicular space must be slower than in the other spe- 

 cies with their relatively small intrafollicular cavities, or, to ex- 

 press it differently, there is relative stagnation in intrafollicular 

 secretorj^ flow in the case of the thj^-oid follicles of Raia. It is 

 well known that an albuminous secretion which is rendered rela- 

 tively stagnant within the epithelial cavities of the body tends to 

 produce colloid masses whose microchemical reactions more or 

 less closely resemble those of the colloid material of the thyroid 

 gland; this occurs, e.g., in the ducts and tubules of the resting 

 mammary gland and in dilated cystic tubules in the kidney. We 

 would therefore expect that in the thyroid follicles of Raia with 

 their relatively stagnant secretory flow we should find an increased 

 amount of colloid material. This I find to be the case, the pro- 

 portionate volume of colloid present in the follicles of Raia being 

 decidedly greater than in the other species. Similarly I find it 

 the rule that the larger follicles contain relatively more colloid 

 than the small follicles in the same gland. The volume of col- 

 loid contained in the thyroid follicles, therefore, can not be 

 regarded as an index of the activity of the secretory epithelium; 

 it would rather appear as a sort of by-product whose volume 

 was dependent upon the rate of flow in the fluid from which it 

 was formed. This view harmonizes the appearance of colloid 

 material in the thyroid gland with the occurrence of similar mate- 

 rial in the other glandular portions of the body, and with those 

 theories of thyroid secretion which regard the colloid as a by- 

 product rather than as the secretion. Moreover the great varia- 

 tions in the amount of colloid in the thyroid follicles are then 

 explicable upon the basis of variations in the rate of secretory 

 flow which, in turn, is dependent upon the physiological factors of 

 blood and nerve supply as well as upon the anatomical factors. 

 It is also interesting to observe that the volume of secretory 

 epithelium in the several species examined remains in each case 

 approximately proportionate to the size of the thyroid gland and 

 to the size of the fish. The relation of the epithelium to the folli- 

 cular content and to the blood vessels and lymphatics seems to me 

 to indicate most clearly that the secretion is poured out from the 

 epithelial cells so as to find its waj^, on the one hand directly into 



