620 



the zone of the ultiraobrauchial bodies, bring these structures in 

 contact with the dorsomedial edge of the thyroid, and the fusion that 

 results initiates a new epoch in the history of the caudal pharyngeal 

 complex. 



The period at which the fusion takes place is doubtless somewhat 

 variable, if the length of the embryo is taken as a criterion. In 

 embryos 11 mm. (Figure 2), 11.5 mm., 13.6 mm., and 14.5. mm the 

 fusion had not yet occurred. In embryos 12 mm. (1), 12 mm. (2), 



13.5 mm., 15 mm. (1) 

 and (2), and all larger 

 embryos the fusion had 

 occurred. The sequence 

 may be given as fol- 

 lows ; — 12 mm. (1), 

 15 mm. (1), 12 mm. (2), 

 13.5 mm., 16.4 mm., 

 15 mm. (2), 18.2 mm., 

 19 mm., etc. the series 

 being arranged accor- 

 ding to the fusion and 

 amount of transforma- 

 tion which the body has 

 undergone. With the 

 thyroid the ultimobran- 

 chial (i. e., the body of 

 the tripartite complex) 

 fuses first upon its ex- 

 ternal and caudal aspect. 

 Subsequent growth of 

 the thyroid surrounds 

 it on all sides save on the dorso-raedial aspect where this derivative 

 remains longest exposed. 



Before fusion occurs the caudal pocket or extension of the 

 ultimobranchial complex is a hollow structure with relatively thick 

 walls (figure 2) with an epithelium (syncytium?) not unlike that of 

 the ventral pocket of the third pouch which becomes the thymus, as 

 it appears at this stage and for some time subsequently. After the 

 fusion, a transformation takes place, appearing at first upon the side 

 where the fusion has occurred. The appearance is quite characteristic; 



Fig. 5. Section througli the caudal pharyngeal 

 complex, both sides, in Embryo No. ä, 19 mm., Gage 

 collection. Photograph, x 40. C carotid artery; 

 L lumen ; P. IV. parathyroid IV ; Th. thyroid ; Z7. ul- 

 timobranchial body. 



