THYREOID GLAND IN SQUALUS ACANTHIAS 201 



alternate swellings and constrictions. But, as noted above, the 

 initial swellings due to this process are only apparent and actually 

 are not greater in thickness than is the plate in other parts of its 

 extent where indentations have not yet occurred. 



The extraordinary cellular activity of the epithelium at this 

 stage is clearly manifested by the large number of mitotic figures. 

 The localization of these mitoses is even more significant than is 

 their number. The nuclear figures are usually found only in 

 those places in the epithelial plates where actual thickenings on 

 the plates are being formed. Therefore the little mounds which 

 appear on the plates, as the ipmediate anlagen of the early 

 follicles, may be formed not only by the rearrangement of the 

 already existing cells of the epithelial plates, but also by the for- 

 mation of new cells as well. Consequently, it can easily be seen 

 how the apparent swellings on the plates, produced by the re- 

 arrangement of the existing cells, may be transformed into 

 actual swellings by the absolute increase in number of the cells 

 found in a localized area. These swellings 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 nd 

 proliferating, they are also increasing in sfze. This progressive 

 increase in the height of the cells corresponds in general to the 

 progressive stages in the differentiation of the two-celled plate 

 into newly formed follicles. So that the thyreoid gland of an 

 embryo 60 mm. long 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 follicle from the epithelial 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 columnar cells, whose nuclei are 

 peripherally placed. This arrangement results in the formation 

 of a striking picture. The nuclei are regularly placed at the 



