MORPHOGENESIS OF THE FOLLICLES 423 



b. Follicular 



A thyroid follicle will be defined as a completely closed sac 

 whose wall is usually made up of only a single layer of epithelial 

 cells. This definition includes all the features of the follicle 

 which may be regarded as absolute and constant. The size and 

 shape of the follicle may vary, and great differences are found in 

 these respects in follicles of the same gland as well as in follicles 

 of fetuses at different stages. Typically the thyroid follicle may 

 be considered spherical or spheroidal in shape; but, as will appear 

 later, this type is subject to considerable variation. The term 

 primary follicle will be used to include those follicles develop- 

 ing independently of preexisting follicles. The follicles derived 

 from preexisting follicles, by budding or otherwise, are termed 

 secondary foUicles. 



In the present series, the first primary thyroid follicles appear 

 in a fetus 24 mm. in crown-rump length (No. 20). In this fetus 

 the thyroid gland has essentially the same structure as has that 

 of the last (No. 18) described in the prefolHcular period, i.e. it 

 is made up chiefly of longitudinally placed epithelial plates or 

 bands, only two cells in thickness. But in this stage, the plates, 

 which have in previous stages been characterized by compara- 

 tively smooth surfaces, now present surfaces which are more or 

 less roughened by the appearance of scattered hillocks or 

 mounds (figs. 13 and 14). They are placed very irregularly 

 with respect to one another, and may appear for the first time 

 in any part of a plate, at its periphery or in a more central 

 region. 



When studied in cross sections (figs. 2 and 3) it is found that 

 these hillocks are the immediate anlages of the early thyroid 

 follicles. It is further seen that the hillocks are apparently pro- 

 duced by the concurrence of four different processes in the epi- 

 thelium. The first process is that of cell rearrangement, the 

 second that of cell proliferation, the third that of absolute cell 

 growth, and the fourth that of lumen formation. These proc- 

 esses, although described separately, may occur simultaneously. 



The first departure from the two-celled plate arrangement, in 

 the process of folhcle formation, is found in a rearrangement of 



