430 EDGAK H. NORMS 



From the time of their first appearance, considerable varia- 

 bility in the size of the lumina found in any particular gland is 

 to be noted. The first isolated follicles are found in the more 

 peripheral parts of the thyroid gland, and it is in these regions 

 that they first attain large size and considerable complexity. 

 For some time after the formation of follicles has begun, all 

 stages previously described may be found in different parts of 

 the same gland, a considerable portion of which retains the 

 irregular plate-like type of structure characteristic of the pre- 

 follicular stages. 



In the foregoing account, the cell masses in which the lumina 

 develop have been described as spherules whose cross section is 

 circular in outline. While this is true for typical follicles and in 

 most cases, some variation within comparatively narrow limits 

 is found. Ovoidal or somewhat irregular foUicles occur, but 

 these are not more numerous than would be expected in a rapidly 

 growing tissue. 



The foregoing descriptions of the early primary follicles have 

 been taken in large part from observations made on two fetuses, 

 one of 24 mm. (No. 20) and one of 30 mm. (No. 30). The ex- 

 cellent condition of these specimens has made possible studies 

 of considerable detail. The members of the series (Nos. 21, 22, 

 23, 24, 25, 26, 27, 28, 29) intervening between these two, al- 

 though not favorable for such intensive studies, show substan- 

 tially similar structure. These stages may be summarized very 

 briefly. The comparatively smooth epithelial plates of the pre- 

 follicular period have been transformed into plates with rough 

 surfaces. The roughenings on the plates are the early indica- 

 tion of the follicles about to be formed. With the progressively 

 increasing number of follicles the plates are transformed into 

 irregular bands, which in turn give rise to groups of sohd or hol- 

 low masses of cells. In the 30 mm. stage, however, the thyroid 

 gland is still largely made up of anastomosing bands (fenes- 

 trated plates) (figs. 3, 6, 13, 14), although some entirely isolated 

 folHcles are found. 



While the fetus is increasing in length from 30 mm. to 50 mm. 

 the thyroid gland presents merely a continuation of the process 



