MOEPHOCiENESIS OF THE FOLLICLES 437 



V. DISCUSSION AND CONCLUSIONS 



Remak's theory of the derivation of the thyroid folhcles di- 

 rectly from the primitive saccular thyroid anlage has not been 

 confirmed. In the prefollicular stages, the thyroid is by recent 

 investigators quite generally described as assuming the form of 

 irregular, anastomosing 'cords' or masses of epithelium. This 

 undoubtedly appears to be the case when sections of the gland 

 are observed (figs. 1 and 2). But the reconstruction methods 

 used in the present investigation reveal a surprisingly different 

 condition. It is found that, as a matter of fact, in the great 

 majority of cases the cords are illusions and in reahty are 

 merely sections of fenestrated epithelial plates longitudinally 

 arranged. 



As to the further steps in the process of morphogenesis of the 

 folhcles from these anastomosing 'cords', widely divergent views 

 have been held, as noted in the section on 'Literature.' While 

 differing considerably in detail, these views may be classified 

 according to their principal features. The morphogenesis of the 

 primary follicles and of the secondary follicles will be considered 

 in their order. 



According to the apparently most widely accepted view, based 

 perhaps "argely upon preconceived ideas or theories concerning 

 the evolution and morphogenesis of the thyroid gland, there are 

 two dstinct stages in the transformation of the epithehal 'cords' 

 into folhcles. First the anastomosing 'cords' acquire lumina, so 

 that the gland becomes a more or less definite network of hol- 

 low epithehal tubes. The tubes then become constricted (by 



Fig. 15 Wax reconstruction of a portion of a peripheral lobule of the thy- 

 roid gland of a human fetus 60 mm. long (No. 40). This model is from the region 

 shown in figure 10. The structure placed across the top of the figure is a blood 

 vessel. The oblique lines designate cut surfaces. Viewed from above. X 270. 



Fig. 16 A series of follicles (a, b, c, d, e) reconstructed by Born's wax-plate 

 method to show the varying degrees of complexity. All are taken from the 

 thyroid gland of a human fetus 86 mm. long (No. 4-4). X 135. 



Fig. 17 Wax reconstruction (Born's method) of a peripheral lobule of the 

 thyroid gland in a human fetus 158 mm. long (No. 49). The variable form and 

 mutual relations of the follicles are evident. The structure at the left of the 

 figure is a blood vessel. The oblique lines designate cut surfaces. X 130. 



