MORPHOGENESIS OF THE FOLLICLES 433 



treiiio complexity of the gland at this stage. The hollow and 

 sohd epithelial masses are about equal in number. The ex- 

 treme variability in the form of the follicles is the most striking 

 feature (figs. 4, 10 and 16), and the reconstructions reveal more 

 clearly the complexity of these structures than is apparent in 

 sections. Elongated follicles with numerous hollow or solid 

 buds, as well as numerous varieties of other more or less com- 

 plex arrangements are to be seen. The whole picture is one of 

 active growth. There is great variability in the size of the fol- 

 licles, the largest being for the most part located in the periph- 

 eral zone ; but the extreme variation in the follicular form makes 

 absolute measurements for comparison of follicles of little value. 



This condition of the gland prevails in the remaining members 

 of the series, up to and including the 158 mm. fetus (Nos. 45, 46, 

 47, 48, 49). There are but two differences to be noted. In the 

 first place, the relative number of follicles is increasing from 

 stage to stage, so that by the time the fetus is 158 mm. in 

 length, the follicles in number are far in excess over the sohd, 

 interfollicular masses. Secondly, the number of irregular and 

 complex forms, although still occurring, is becoming fewer (fig. 

 17), and the spheroidal follicles are relatively numerous. 



The thyroid of the 163 mm. fetus (No. 50) has quite a different 

 structure from that described for the members of the series just 

 preceding (Nos. 44, 45, 46, 47, 48, 49). The irregular and 

 branching follicular complexes are comparatively few in num- 

 ber, while small, spherical follicles make up almost the whole 

 of the tissue. There are, however, a few solid, interfollicular 

 epithehal masses still present at this stage. The picture has 

 changed from one in which the epithehal structures, instead of 

 being complex in their form, have become relatively simpler in 

 character and for the most part are organized into small 

 folhcles. 



My material from this stage on up to and including newborn 

 children shows to a greater or less extent the process of epithe- 

 lial desquamation in the thyroid follicles as described by Elkes 

 ('03), Hesselberg ('10), Isenschmid ('10) and others. Whether 

 this process is physiological or pathological is as yet undeter- 



