210 E. V. COWDRY 



the thyroid may be due to the lack of nervous influence, though this is 

 denied by Kummer. 



Embryology 



It would seem, from the recent work of Norris (&), that the conflicting 

 statements with regard to the normal mode of development of the thyroid 

 gland are merely expressions of the wide latitude of variation. 



The thyroid gland begins to develop very early (embryos of about one 

 millimeter and a half) as a median evagination of the endoderm of the 

 floor of the pharynx between the first and second pair of pharyngeal 

 pouches (see Figures 2 and 3 of section on Parathyroids), there being no 

 regular contributions from the lateral branchial clefts, as was formerly 

 supposed. 



Early Development. At this stage considerable variation becomes 

 apparent, and Norris illustrates his account by means of the accompany- 

 ing diagram (Fig. 3). Development may take place in three directions: 

 the downgrowth may be solid (c), or possess a hollow stalk (d), or be 

 double (e). The last named condition is rare, and Norris says that it 

 "would seem to support" Patten's idea that the thyroid may be the repre- 

 sentative of two separate glands associated with the pharynx in certain 

 lower forms. 



The stalks connecting the glands with the pharynx now elongate, so 

 that the conditions represented in (g), (h) and (i) are brought about. 



With complete separation from the pharynx, in embryos of from about 

 four to seven millimeters, a number of variations arise, as is illustrated 

 in diagrams (j) to (q). Division may occur near the pharynx, leaving 

 a large amount of the tissue of the stalk in association with the thyroid 

 body, thus favoring the formation of a pyramidal lobe or thyroglossal 

 duct ; and, conversely, the development of "lingual rests" and "suprahyoid 

 bodies" results when separation takes place near the body of the thy- 

 roid, leaving a larger amount of tissue in association with the phar- 

 ynx. 



Cavitation. The next change consists in the formation of closed 

 cavities within the gland mass (r), which, according to Norris, are quite 

 independent of the original lumen of the thyroid pouch. He attributes 

 considerable importance to these intraglandular spaces, because he has 

 also found them in approximately the same stage of development in 

 representatives of several classes of animals. These spaces finally open 

 to the periphery of the gland anlage (s), and the definitive follicular 

 cavities begin to appear in their walls (t). 



Colloid Formation. It is not until after birth that any considerable 

 amount of colloid material is formed within the follicles, and iodin may 



