MORPHOGENESIS OF THE FOLLICLES 443 



gested that these were originally sohd buds whose lumen ap- 

 peared independently and later established a secondary con- 

 nection with, the lumen of the parent foUicle. While it is diffi- 

 cult to disprove such an occurrence, it would tend to reduce 

 rather than to increase the number of follicles and seems im- 

 probable. The parent follicle apparently sends off extensions 

 or branches, which represent both the wall and the lumen and 

 gradually become constricted off to form new follicles. All pos- 

 sible stages in such a process, from the sHghtest outpouching of 

 the wall to the finally separated follicle are easily observable. 



3. The third method of secondary folhcle formation is the 

 simple division of the parent follicle somewhat as described by 

 Peremeschko. The process appears similar to that of hollow 

 bud formation. The follicle first takes on the form of an elong- 

 ated elhpse which becomes constricted about its center. No 

 cases of division by the ingrowth of epithehal spurs as de- 

 scribed by Isenschmid (for postnatal stages) have been observed 

 in my fetal material, though such may possibly occur. 



Ha^dng considered the primary and secondary folhcles sepa- 

 rately it remains to consider them in their relations to each 

 other and to the gland as a whole. 



The formation of the secondary thyroid follicles begins when 

 the fetus has reached a length of about 56 mm. From the 70 

 mm. stage on the formation of secondary follicles progresses 

 \\Tith such rapidity that the total number of follicles is very 

 greatly increased within a short period of time, and the relative 

 number of primary folhcles becomes progressively smaller. 



It might be thought that such elongated, irregular, branched 

 and budding forms as appear in large numbers in certain of the 

 older members of the series (especially in the 86 mm. fetus, No. 

 43) are derived from irregular, branching bands (fenestrated 

 plates) retained from the prefollicular period. But the evi- 

 dence disproves that view. After the appearance of the first 

 follicles, the bands and plates are being progressively broken 

 up until the gland parenchyma in a fetus of 65 mm. is practi- 

 cally devoid of such structures, and is almost entirely made up 

 of isolated sohd or hollow masses of cells. So closely does this 



