THYEEOID GLAND IN SQUALUS ACANTHIAS 215 



even in those forms in which such a structure is sometimes devel- 

 oped, there are no specific cases described in which the Uimen of 

 the thyreoglossal duct is continuous with a cavity in the gland- 

 mass proper. In other words, since the lumen of the duct does 

 not extend into the body of the gland, any cavities found there 

 must be morphologically independent of it, although they may, 

 as the theory would suggest, be phylogenetically related. 



The development of intra-epithelial clefts and spaces is not 

 unusual. The lumina of a number of the true glands are formed 

 within solid epithelial sprouts and buds which are the anlagen of 

 the future ducts and tubules. /Again, the vacuoles in the epi- 

 thelium of the oesophagus and duodenum which were described 

 in detail by Johnson ('10) are striking examples of this same fea- 

 ture of morphogenesis. In all these instances, however, the 

 spaces ultimately form a connected system of cavities or open into 

 a common lumen. The intra-epithelial cavities of the early 

 thyreoid gland, on the other hand, have a very different fate. 

 They open more or less independently of one another to the out- 

 side of the gland mass and are then invaded by the surrounding 

 vascular mesenchyme. Such a process apparently has not been 

 observed in the morphogenesis of any other organ and seems to 

 be quite unique and pecuUar to the thyreoid gland. 



Before attempting to discuss the follicular period, the fact 

 must be emphasized that the cavities or lumina of the thyreoid 

 folUcles are entirely independent of the earlier transient intra- 

 glandular cavities described in the present paper. 



Remak's ('55) theory of the derivation of the thyreoid follicles 

 directly from a primitive saccular thyreoid anlage has not been 

 confirmed. In the prefollicular stages, the thyreoid 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. 11, 12, 13). But the reconstruction methods used 

 in the present investigation reveal a surprisingly different con- 

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

 of cases the cords are illusions and in reality are merely sections 

 of fenestrated epithelial plates longitudinally arranged. 



