202 E. H. NORRIS . 



periphery of the circle and form a dark band, which surrounds a 

 clear, central cytoplasmic portion. The magnitude of this cyto- 

 plasmic area and the sharp contrast between the two portions 

 (in the stained preparations) are usually prominent features. 

 The lumen makes its appearance in the center of this cytoplasmic 

 area as a tiny spherical space outlined by a definite and regular 

 margin. It is as though the cells had but drawn apart a little, 

 so that their central ends, instead of remaining in contact with 

 one another, are separated by an interval. It is important to 

 note that no tubular stage is found in the process of lumen for- 

 mation, but, as was pointed out in the case of the human thyreoid 

 gland (Norris, '16), the lumina appear as absolutely independent 

 spaces. 



When the lumina first appear they have no apparent content; 

 but undoubtedly they contain some substance which is not 

 stained with the ordinary method and which increases in amount 

 with the size of the follicular cavity. Certain of the larger lumina 

 (not all of them), which are perhaps older, are found to contain 

 a hazy, granular substance. Typical colloid does not appear 

 until later, in the 160-mm. stage (No. 50). 



In the foregoing account, the cell masses in which the lumina 

 develop have been described as spherules whose cross-section is 

 circular in outline. While this is true for typical follicles, some 

 variation, within comparatively narrow limits, is found. Ovoidal 

 or somewhat irregular follicles occur, but these are not more 

 numerous than would be expected in a rapidly growing tissue. 



These stages, which have just been described, may be sum- 

 marized very briefly. The comparatively smooth epithelial 

 plates of the prefollicular period have been transformed into 

 plates with rough surfaces. The roughenings on the plates are 

 the early indication of the follicles about to be formed. With 

 the progressively increasing number of follicles the plates are 

 transformed into irregular bands, which in turn give rise to groups 

 of solid or hollow masses of cells. 



Although the first well-formed primary follicles are found 

 in embryos about 60 mm. long, the epithelial plates of the pre- 

 follicular period are only very gradually transformed and re- 



