MORPHOGENESIS OF THE FOLLICLES 427 



epitheliiil plates whei-e iictiuil thickenings on tlio plates are 

 being" formed. Tlierefore the little mounds which appear on 

 the plates, as the immediate anlages of the early follicles, may 

 be formed not only by the rearrangement of the already existing 

 cells of the epithelial plates, but also bj^ the formation of new 

 cells as well. Consequently, it can easily be seen how the ap- 

 parent swellings on the plates, produced by the rearrangement 

 of the existing cells, m.B,y be transformed into actual swellings, 

 by the absolute increase in number of the cells found in a local- 

 ized area. These swelhngs become roughly spheroidal in form. 



The third process referred to above is the absolute increase in 

 size of the cells. While the cells are shifting their axes and pro- 

 liferating, they are also growing in size. This fact results in 

 the appearance seen in figure 2, where the sohd, two-celled 

 plates are found in some cases to be no greater in cross section 

 than the one-celled plate which surrounds the follicle lumen. It 

 might be thought that the cells do not actually increase in size, 

 but only increase in height by a closer crowding together. But 

 a study of figure 3 will show that such is not the case. For the 

 cells are not more closely packed together in the newly formed 

 follicles than they are in the two-celled plate. 



This progressive increase in the height of the cells corre- 

 sponds to the progressive stages in the differentiation of the two- 

 celled plate into newly formed follicles. So that the thyroid 

 gland of a 30 mm. fetus (No. 30) presents in different regions 

 epithelial cells varying greatly in height. The lowest cells are 

 found at the beginning of the process, in the two-celled plate; 

 the highest being found at the other extreme, in the completely 

 formed follicle. 



Three of the four processes above mentioned as apparently 

 involved in the evolution of the folhcle from the epithehal plate 

 have now been reviewed in detail. The formation of the lumen 

 remains to be considered. Just preceding the appearance of the 

 lumen, the spherule (in which it is about to develop) appears in 

 cross section as a circlet of very tall columnar cells, whose 

 nuclei are peripherally placed. This arrangement results in the 

 formation of a striking j^icture. The nuclei are regularly placed 



