94 J. A. BADERTSCHER 



dal halves lie at varjdng distances dorsal to the lateral margin 

 of the thyroid gland which has the general shape of a crescent 

 with its horns directed anteriorly and dorsally. In a few sec- 

 tions they are separated from the thyroid only by a very thin 

 layer of connective tissue iU). They extend almost to the cau- 

 dal margin of the thyroid gland. Their caudal ends lie more 

 closely together than their anterior ends. 



The ultimobranchial bodies at this stage are composed of a 

 syncytium. No cell walls are present. Vacuoles are found 

 throughout their entire extent, although their distribution is not 

 uniform. In places they can be found throughout an entire 

 cross section of an ultimobranchial body while in other places 

 they are confined to its more central portion. The vacuoles vary 

 in size, the largest being almost as large as some of the nuclei. 



The nuclei vary somewhat in size and in shape. Some are 

 oval, some round, while others are irregular in outline. They 

 contain from one to three nucleoli and a rather generous amount 

 of chromatin which is in the form of granules and threads. The 

 more centrally located nuclei have no regular arrangement while 

 those near the periphery are in places quite regularly arranged. 

 They are more closely packed together in the nonvacuolar than 

 in the vacuolar portions of the ultimobranchial bodies. A con- 

 sideration of this feature is of particular importance in stages in 

 which the ultimobranchial bodies have fused with the thyroid 

 gland. Mitoses of the nuclei can readily be found, especially in 

 the larger more caudal part of the bodies, thus indicating a 

 growth tendency of these structures. Neither blood vessels nor 

 connective tissue are present in the ultimobranchial bodies at 

 this stage. 



The thyroid {T) is composed of nonvacuolar cell masses and 

 cell cords^ the latter of which are for the most part transversely 

 arranged. No cell walls can be demonstrated, hence the cell 



■* Norris ('16) finds that in early developmental stages of human embiyos the 

 cell cords seen in cross sections of the thyroid gland represent in reality sections 

 of fenestrated epithelial plates. As I have not made a careful study of the for- 

 mation of the follicles in the thyroid gland I shall use the term 'cell cords' which 

 is the microscopic picture presented in cross sections of the gland. 



