288 DAVID MARINE 



growths may at present be called benign tumors, they are at most only 

 partial tumors since they have retained many of the physiological char- 

 acteristics of normal thyroid. Adenomas, like fibroids of the uterus, are 

 rarely seen except in man, and on this account, what should be the most 

 favorable tissue for the experimental study of the general tumor problem, 

 cannot be utilized. The nearest approach to adenoma formation in the 

 lower animals is found in the thyroid of the white rat, but these circum- 

 scribed areas of hyperplasia are composed of well differentiated thyroid 

 which react readily with iodin, and therefore are properly considered as 

 part of the general physiological overgrowth. Adenomas most frequently 

 occur in simple or endemic goiter about the age of puberty. They are 

 also seen in congenital goiter and may develop, though rarely, late in life. 

 They are rarely ever seen apart from goiter and therefore the sex inci- 

 dence is the same as that of goiter. They may be solitary but are more 

 frequently multiple. The view that they arise from cell rests which are 

 present in most normal thyroids offers the best explanation of the origin 

 of these growths. These cell rests may be considered as excess thyroid 

 tissue beyond the needs of the organism which are slowly being destroyed, 

 but when the stimulus for increased thyroid activity reaches the gland all 

 the viable cells, including these cell rests, react with growth, and the 

 type of growth morphologically depends on the degree of differentiation 

 of the cells at the time the stimulus was applied. This hypothesis would 

 account for the wide range of cell types from fetal adenoma to those 

 indistinguishable from simple hyperplasia. In addition to the above 

 range of morphological variations dependent upon the degree of differ- 

 entiation of the cells at the time the adenomas start to develop, one must 

 also recognize that these growths are capable of differentiation during 

 their development, that is, they have a cycle of cell changes (growing, 

 involuting and colloid or resting stage) similar to that of the normal 

 thyroid. These may be represented in the following scheme : 



Atrophy 

 / 

 Thyroid rest -> Hypertrophy - Hyperplasia 



\ 

 Involution 



\ 



Colloid or 

 resting. 



The particular morphology, therefore, of any given adenoma is in part 

 dependent on the degree of differentiation of the cells from which the 

 tumor arose, and in part dependent on the stage in the cycle of cell 

 Changes existing at the time of examination. A simple and satisfactory 

 anatomical classification of these growths cannot be made. In order to 



