422 EDGAR H. NORRIS 



manent thyroid tissue is still uncertain. This region is there- 

 fore purposely excluded from the following descriptions, which 

 apply primarily to the process of morphogenesis in the struc- 

 tures derived from the median thyroid anlage. 



The thyroid gland in the final stage of the prefollicular period 

 is shown in a fetus of 22 mm. (No. 18). This stage will be de- 

 scribed in some detail, in order to make clear the subsequent 

 process of morphogenesis of the thyroid follicles. When studied 

 in sections, the thyroid at this stage is seen to be made up appar- 

 ently of epithelial 'cords' which are interrelated in a complex 

 manner (fig. 1). A loose, anastomosing network is thus formed, 

 the interspaces of which are filled with a vascular mesenchymal 

 tissue (fig. 1). The 'cords' are, in general, only two cells in 

 width, a feature characteristic of the thyroid plate and its 

 derivatives at various stages. 



Upon reconstruction (figs. 11 and 12) it is found that the epi- 

 thelial network seen in sections, and described by so many ob- 

 servers as consisting of 'rods' or 'cords,' is merely a section of 

 flat, slab-Hke plates, or bands, representing portions of fenes- 

 trated plates. It is true that some few of the epithelial masses 

 are actually cord-like in form, but by far the greater number 

 are better described as bands or plates. A definition of terms 

 is necessary at this point. The term 'plate' will be used to sig- 

 nify a structure of relatively slight thickness, presenting expan- 

 sive surfaces which are more or less smooth, and which may or 

 may not be perforated (fenestrated). A band is a narrow 

 plate. Therefore a fenestrated plate may be considered as 

 made up of a number of anastomosing bands. There is great 

 variability in the way in which these plates and bands are 

 arranged. Some are mere slabs, which may or may not be per- 

 forated; others form irregular prisms or rounded cylinders, 

 which open at both ends into the surrounding mesenchyme. 

 The length and width of these plates is also quite variable. In 

 three respects, however, they are in general agreement. They 

 are two cells thick; present fairly smooth surfaces; and are 

 longitudinally placed. Within these epithelial bands (fenes- 

 trated plates) the primitive follicles of the thyroid gland develop. 



