MOKPHOCiENESIS OF THE FOLLICLES 431 



abo^•e described; so that by the time the 50 iniii. stage is I'eached 

 the band oi' plate formations are relatively insignificant, while 

 the isolated epithehal masses make up the greater part of the 

 organ. All stages in the breaking up of the bands or plates may 

 still be found, however. At this stage (50 mm.) the gland is 

 therefore characterized by the presence of few epithelial plates 

 or bands, very many isolated, solid cell-masses (anlages of fol- 

 licles) and relatively few well developed follicles. 



At 56 mm. (No. 39) the thyroid gland presents shghtly more 

 folhcles than at 50 mm. Not only are the follicles increased in 

 number, but they also show some changes in both size and form. 

 In the previous stages the follicles have ranged from 16 to 55 /i 

 in diameter (average diameter about 40 /j.), and have been typi- 

 cally spherical in form. But in this stage many of the follicles 

 at the periphery of the gland have enlarged considerably and 

 have departed from the more usual spherical form of previous 

 stages. This condition is more strikingly seen in the next fetus 

 of the series (60 mm., No. 40) where both increase in size and 

 change in form of the folhcles are very evident (fig. 15). The 

 number of follicles at this stage appears about equal to the num- 

 ber of soUd epithehal masses. Figure 7 is a graphic recon- 

 struction of a number of epithelial masses from which folhcles 

 are being formed and of certain isolated folhcles. A study of 

 sections at this stage will show folhcles situated at the periph- 

 eral portions of the lobe whose diameters are three or even four 

 or five times as large as those of greatest magnitude located 

 more centrally. The average diameter of the centrally placed 

 folhcles is about 15 microns, while the average diameter of the 

 folhcles in the peripheral regions is approximately 55 microns. 

 There is much less absolute variation in the size of the central 

 folhcles than in those of the periphery. Centrally placed fol- 

 hcles range from 13 to 17 microns, while peripherally placed 

 follicles range from 20 to 103 microns in diameter. It will 

 further be seen that these same large peripheral folhcles -are 

 quite irregular in form. Instead of being circular in outhne, 

 they have a more or less regularly elliptical or ovoidal form, and 

 may present a number of solid or hollow buds. Figure 9 rep- 



