MORPHOGENESIS OF THE FOLLICLES 445 



VI. SUMMARY 



By methods of reconstruction (both graphic and wax-plate), 

 the compHcated process of morphogenesis of the folHcles of the 

 prenatal human thyroid gland has been worked out and several 

 mooted points definitely estabhshed. 



1. The so-called 'cords' forming the anastomosing network in 

 sections of the thyroid (median anlage) in the later prefollicular 

 stages represent chiefly sections of epithelial bands, two cells in 

 thickness, and forming irregular, fenestrated plates. 



2. The frequently described stage in which the 'cords' are 

 transformed into an anastomosing set of epithelial tubes from 

 which the follicles are derived does not exist. The process of 

 follicle formation gives no e\adence or indication that the th5Toid 

 has been derived from a branching tubular gland. 



3. The primary thyroid folhcles arise directly as isolated and 

 independent structures from the epithelial plates of the pre- 

 folHcular period, by the rearrangement of cells, cell proliferation, 

 increase in the size of the cells, and lumen formation. 



4. The primary folUcles appear in fetuses about 24 mm. in 

 length. The epithehal bands (fenestrated plates) have prac- 

 tically disappeared in a fetus of 65 mm., but a few solid inter- 

 folHcular epithelial masses are still present in fetuses 163 mm. 

 in crown-rump length. 



5. Secondary thyroid follicles are formed from preexisting 

 follicles apparently by three methods: by solid buds; by hollow 

 buds; and by constriction of the parent follicle. 



6. The first secondary follicles appear in fetuses about 56 mm. 

 in length, but are formed most rapidly in stages when the fetus 

 is between 80 mm. and 158 mm. long. After 163 mm., the 

 growth of the gland probably takes place largely by the increase 

 in size of the individual follicles, rather than by increase in 

 theii" number. 



7. Large cystic folhcles were observed in the lower and pos- 

 terior (dorsal) parts of four glands from the older fetuses. Their 

 significance is uncertain, as is likewise the apparent involution 

 of the follicles with desquamation of epithelium observed in the 

 later fetal and newborn stages. 



