420 EDGAR H. NORRIS 



IV. MORPHOGENESIS OF THE THYROID FOLLICLE 

 a. Prefollicular period 



A brief consideration of the thyroid gland in the prefolHcular 

 period is essential to an understanding of the folHcular develop- 

 ment. For it is during this prefollicular period that the anlages 

 of the primitive follicles are derived from the original epithelial 

 mass; and, to a certain extent at least, the size, form, and ar- 

 rangement of the earlier follicles are thereby predetermined. 



The earher well-known stages in the development of the 

 (median) thyroid anlage in the human embryo are not consid- 

 ered in the present paper. The original thyroid diverticulum 

 becomes detached and transformed into a small epithelial plate, 

 well shown in the 6 mm. embryo (No. 1 of the present series). 

 This and several succeeding prefollicular stages of the thyroid 

 were carefully reconstructed by Born's wax-plate method, but 

 for the present purposes it is unnecessary to figure or describe 

 these models. 



As is well known, the (median) thyroid epithelial plate soon 

 presents irregularities, as shown in the 7.5 mm. embryo (No. 2), 

 and rapidly becomes transformed into what appears in cross- 

 sections to be (as heretofore almost universally described) a 

 network of anastomosing epitheUal cords. 



With the details of this process of transformation the present 

 study is not concerned. One feature of the end result, however, 

 which comes out clearly in the reconstructed models, is that the 

 cord-like appearance seen in the sections is largely an illusion. 

 Fundamentally the plate-like structure of the thyroid anlage 

 persists for a considerable time, although somewhat modified by 

 a compUcated process of fenestration, splitting, and budding 

 during the growth of the primitive epithelial plate. The re- 

 sultant structULre consists essentially in a mass of irregular, 

 branching and fenestrated plates, for the most part longitudinally 

 arranged (parallel to the long axis of the body), so that in cross- 

 section they appear as 'cords' of epithelial cells (fig. 1). This 

 type of structure, with varying degrees of complexity is found 

 in the various prefollicular stages of embryos from about 10 mm. 



