MORPHOGENESIS OF THE FOLLICLES 441 



late them from the parent plate. The vascular mesenchyme 

 doubtless takes some shght part in the process, however. 



As previously pointed out, the primary foUicles may not in 

 every case be at once separated completely from the plate. 

 Instead of being in all cases sharply outlined and spheroidal in 

 form, small portions of the p\ate (epithelial tags) may be 'eft 

 hanging to the foUicle wall. The significance of such epitheUal 

 tags is readilj^ understood when it is considered how easily they 

 might be mistaken for epithehal buds arising from the primary 

 follicle in which secondary follicles were about to form. These 

 structures will be mentioned later in the discussion of the sec- 

 ondary follicles. It has been seen that there is no sharp fine of 

 demarcation between the prefollicular and folhcular periods. 

 Similarly, in the origin of follicles, the period of primary follicle 

 formation is not sharply marked off from that of secondary fol- 

 licle formation. 



The number of thyroid follicles is apparently not absolutely 

 established or finally limited at any stage of development (as is 

 the case, for instance, with the glomeruli of the kidneys). In 

 the earher stages, the number of follicles increases by the forma- 

 tion of additional primary follicles. Soon, however, these pri- 

 mary follicles begin to give rise to secondary foUicles (at 56 mm.). 

 In later stages the formation of primary follicles apparently 

 ceases, although their occurrence even in the adult has been 

 claimed, e.g., by Hiirthle ('94) and Sobotta ('15), the new- 

 formed follicles being all secondary in character. Various 

 methods of secondary follicle formation have been described. 



1. Origin fro7n buds or sprouts. Ribbert ('89), L. R. Miiller 

 ('96), Streiff ('97), Isenschmid ('10) and others have described 

 this process. The bud is usually described as a local thickening 

 of the follicle wall, which contiiyiues to increase in size by the pro- 

 liferation of cells, until a solid bud, projecting into the stroma, 

 is formed. Directly, through the concentric rearrangement of 

 the cells, the form of the lumen can be made out, even while the 

 young follicle is still in contact with the mother-follicle. 



2. Originfrom collapsed follicles. Biondi ('89) Anderson ('94), 

 and others have described the process as follows. After fiUing 



THE AMERICAN- JOURXAL OF ANATOMY, VOL. 20, NO. 3 



