622 



i'.- 



thyroid (figure 1) but evideutly proceeds rapidly after fusion has oc- 

 curred. Figures 3, 4, 6 and 7 are given in illustration of successive 

 stages which the body undergoes. Figure 5 (19 mm.) is of a stage 

 intermediate between those of figures 4 and 6, Avhile the condition in 

 the 19 mm. embryo described by Gkosser as " an irregular group of 

 small cells with large strongly stained nuclei " (p. 547) seems somewhat 

 more advanced than that of the 19 mm. embryo shown in figure 5. 

 Figure 6 is of the last stage in which the ultimobranchial body 

 is clearly outlined. The structure at this stage is not easy to interpret. 

 The nuclei are larger and clearer, and I have been unable to deter- 

 mine whether they are 'thyroid' nuclei or 'ultimobranchial' in origin. 



Small or dark nuclei have largely at 

 least disappeared and mesenchymal and 

 vascular elements have invaded the 

 structure. 



Succeeding stages show a poorly 



V^i-' 'Ä^^tf^^^l^fK'''i &'%'■"- circumscribed area of denser tissue 

 «4^'- • m^.^i'tBKa^SiW'jti ^'^^ x-.'" occupying this region, which Tourneux 



and Verdun '97 recognized as derived 

 from the ultimobranchial (postbranchial) 

 body (24 mm.). Grosser rejects this 

 and states that the inner condensation 

 at the 'vasculo-stromal hilus' (Prekant) 

 represents simply a center of thyroid 

 proliferative growth. It appears to me 

 however that the condensation, shown 

 in figure 7, marks the place of disap- 

 pearance of the ultimobranchial body, 

 although it may also well be as Grosser 

 has stated, a center of growth. The 

 correctness of this interpretation is 

 further borne out by the persistence in the embryo 31 mm. (1) of a 

 connection with the inner condensation of an epithelial cord lying 

 outside the thyroid which from its structure and position is clearly 

 derived from the caudal pharyngeal complex. In the 37 mm. embryo, 

 furthermore, an epithelial vesicle, a remnant of the caudal pharyngeal 

 complex, is connected with the condensation (figure 8), — which at this 

 stage is becoming less distinct, — and also by a delicate epithelial cell 

 cord is joined to the parathyroid lY. Beginning with the 41 mm. the 



Fig. 7. Section to show the 

 inner condensation (ultimobranchial 

 body) within the thyroid. Section 

 No. 482, Embryo No. 913, 30 mm., 

 Harvard collection. Photograph, 

 X 60. C. inner condensation (ul- 

 timobranchial body); Th. thyroid. 



