198 E. H. NOKRIS 



are well formed and easily seen some time before the cavities have 

 opened to the outside. 



The second process consists of a change in form of the nuclei. 

 The centrally located spheroidal nuclei become ovoidal like those 

 occurring in the periphery. 



The third process is very closely allied with the second and has 

 to do with the rearrangement of the nuclei. The peripherally 

 disposed nuclei maintain the position they have constantly 

 held, while those of the central part of the gland which have 

 become ovoidal, instead of remaining irregularly scattered, now 

 assume a definite relation to the developing cavities. As a 

 result of their rearrangement, their long axes ^re placed per- 

 pendicular to the margins of the cavities. By this process 

 the vast majority of the nuclei in the thyreoid establish a definite 

 relation to some glandular surface, whether external or internal. 



The fourth process, that of cell proliferation, is, as already 

 mentioned, remarkably rapid during this period. 



As a result of these processes, the gland is changed from the 

 form of a solid epithelial column into a number of epithelial 

 plates. These plates, which are two cells in thickness, possess 

 surfaces which are quite smooth and which are very intimately 

 related to the blood vascular system. It is from these plates that 

 the primary follicles develop. 



4. FOLLICULAR PERIOD 



A thyreoid follicle may be defined as a completely closed sac 

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

 cells. As pointed out in a previous paper on the morphogenesis 

 of the follicles in the human thyroid gland (Norris, '16), 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 embryos at 

 different stages. Typically the thyreoid follicle may be con- 

 sidered spherical or spheroidal in shape; but, as will appear later, 

 this type is subject to considerable variation. The term primary 



