MORPHOGENESIS OF THE FOLLICLES 439 



length is not recorcletl, Init is ])la('('(l in series between fetuses 

 of 10.5 and 22.0 mm. in lenj2;tli. Kingsbury ('15) described fol- 

 licles in a fetus of 32 mm. ; Toiu-neux and Verdun ('97) in one of 

 32.4 mm.; and Grosser ('12) and Sobotta ('15) in fetuses of 50 

 mm. These are the only cases found in the literature where 

 the presence of early follicles has been noted in fetuses of defi- 

 nite length. Several observers refer to the age of fetuses in 

 which the thyroid folhcles appear, but in terms too indefinite 

 to be of value for comparison. 



Although by definition the prefollicular period ends abruptly 

 with the appearance of the first follicles, it is not true that the 

 structure of the gland undergoes any corresponding sudden 

 change with the advent of the folhcular period. The epithelial 

 bands (fenestrated plates) are only gradually replaced by the 

 primary follicles and structures characteristic of the prefollicular 

 period may be present through a considerable part of the fol- 

 licular period, at least until the fetus has attained a length of 

 65 mm. 



Concerning the first three processes (rearrangement of the 

 cells, cell proliferation, and increase in the size of the cells) in- 

 volved in the development of the follicles from the epithelial 

 plate, no further discussion is necessary. The fourth process, 

 however, that of lumen formation, calls for further considera- 

 tion. The folhcular lumen might arise in various ways, which 

 have been suggested by earlier investigators. Most of the 

 workers, however, do not mention the process b}^ which the 

 lumen of the follicle is formed. 



Wolfler ('80) and Lustig ('91) have described the formation 

 of lumina in the sohd cell masses by a degeneration of the more 

 centrally placed cells. The present investigation does not sup- 

 port this view, however. In the first place none of the so-called 

 central cells have been found; and secondly no evidences of 

 degeneration have been observed in the primary follicles. 



It might be supposed, as Hlirthle ('94) and Anderson ('94) 

 have suggested, that the lumen is formed by the accumulation 

 of colloid between the angles of the cells which compose the solid 

 mass. Such a process would leave a colloid-containing space 



