623 



condensation is no longer clearly recognizable. In as much as Grosser 

 speaks of the denser arrangement in the lateral lobes of the thyroid 

 as occurring in embryos of about 50 mm. length, it is possible that 

 he had in mind something different from the inner condensation 

 referred by Tourneux and Verdun and myself to the ultimobranchial. 

 Despite careful study, I have been unable to satisfy myself as to 

 the actual fate of the material so included in the medial portion of 

 the lateral lobes of the thyroid. It is possible that at the stage of the 

 25 mm. embryo (figure 6) ultimobranchial nuclei and cytoplasmic 

 syncytium have entirely disappeared by degeneration, and that only 

 thyroid elements (in addition to 

 the mesenchymal and vascular 

 structures) remain at this period 

 or subsequently at the stage of the 

 inner condensation (figure 7). 

 This is the interpretation that 

 seems to me probably the correct 

 one. The dense staining and small 

 and variable size of the ultimo- 

 branchial nuclei at the time when 

 their source is still clearly recog- 

 nizable ; the suggestion of a be- 

 ginning degeneration in their ap- 

 pearance, and the 'growth behavior' 

 of the unmistakable (median) 

 thyroid cords, together with the 

 circumstantial comparative evi- 

 dence as adduced by Grosser ; — 

 all indicate strongly that the caudal 

 pharyngeal complex does not par- 

 ticipate in the formation of thyroid 

 follicles. On the other hand, it 

 must be confessed that conclusive 



evidence of the fate of the ' so-called lateral thyroid anläge ', ultimo- 

 branchial body, or postbranchial body, as variously termed, is still to 

 be furnished by a histogenetic study with the application of special 

 fixing and staining methods. For this some other mammalian form 

 than. man must be chosen, and until this study is made, we cannot 

 exclude the possibility that some of the colloid containing follicles 

 within the thyroid ma}^ have this source of origin. 



Fig. 8. Section through the lateral 

 lobe of the thyroid to show the connection 

 of the inner condensation with an epi- 

 thelial remnant of the caudal pharyngeal 

 complex. Section No. 521, Embryo No. 820, 

 37 mm., Harvard collection. Photograph, 

 X 60. C condensation ; E epithelial 

 remnant, L lumen of epithelial remnant. 



