621 



the nuclei become dispersed or scattered, the separation of the nuclei 

 being attended by a reticulation of the epithelium (figures 3 and 4) 

 which is clearly syncytial 15 mm. (2), seemingly not unlike the 

 change in the entoderraal thymus (III) at a later stage. The nuclei 

 are, however, much smaller and stain more intensely than the thymus 

 nuclei or the nuclei of the thyroid. They are, furthermore, somewhat 

 irregular in size, many of them markedly pyknotic (12 mm. (2), 

 15 mm. (2), 16 mm.). The fusion is an intimate one, the thyroid 

 elements being directly fused with the ultimobranchial elements 

 (figure 3), the condition being 

 not merely one of envelop- 

 ment on the part of the 

 thyroid, no mesenchymal ele- 

 ments being between the two 

 epithelia. This aj)plies to the 

 earlier stages of fusion ; sub- 

 sequently it becomes more 

 difficult to determine the 

 exact relations, there suc- 

 ceeding a stage in which the 

 area is more circumscribed 

 (21 mm., 25 mm., figure 6). 

 The 'loosening' or reticulation 

 of the ultimobranchial extends 

 until the entire structure is 

 altered in this way. The 

 cavity persists for a time 

 (19 mm. (1), (2); figures 4 

 and 5) but finally is lost. 

 The tubular stalk or neck 

 which joins the body to the 



parathyroid IV may persist for some time but is ultimately broken. 

 Epithelial cords derived from the caudal pharyngeal complex outside 

 of the thyroid and surrounded by mesenchyme were found in two 

 instances extending caudally upon the dorsal or lateral aspect of the 

 thyroid (18.2 mm., 31 mm. (1). It is suggested as highly probable 

 thatsucli epithelial derivatives may subsequently undergo transformation 

 into thymus bodies (IV). The 'loosening' or reticulation of the epi- 

 thelial syncytium may begin apparently before the fusion with the 



-^l-Thyr 



fv'Fig. 6. Section to show a later stage in 

 the transfoimation of the ultimobranchial bodj^ 

 within the thyroid. Embryo No. 29, 25 mm., 

 Cornell collection. Photograph, x 50. Th. 

 thyroid ; Thyr. thyroid cartilage ; TJ. ultinio- 

 brancliial bodv. 



