619 



portion of the complex which we have been speaking of as the ulti- 

 mobranchial body. It is impossible clearly to determine regional 

 values and boundaries in 

 the alternations of shape; 

 the caudally directed end 

 or portion is clearly ultimo- 

 branchial. In all three 

 portions there is a cavity, 

 — a remnant of the original 

 extension of the pharyngeal 

 cavity. Figures 1 and 2 

 may be compared, repro- 

 ductions of sections (10 p.) 

 from the 11. mm embryo 

 eigth sections intervening. 

 At the time of its first 

 appearance, the ultimobran- 

 chial body is morphologi- 

 cally caudad of the thyroid. 

 The relatively rapid descent 



Fig. 3. Early stage of the fusion of the ultimo- 

 branchial body with the thyroid. Section No. 323, 

 Embryo No. 816, 12 mm. Harvard collection. 

 Photograph, X 150. L. Lumen of the body; R. re- 

 ticulation ; Th. thyroid. 



.••^V>K.'...l«V?*?'<.' 



Fig. 4. Section showing a later stage of trans- 

 formation of the ulti mobranchial body. SectionNo.237, 

 Embryo No. 819, 19 mm., Harvard collection. Photo- 

 grapli, X 150. L lumen ; B. reticulation ; Th. thyroid. 



of the latter, following the 

 bifurcation of the truncus 

 aorticus, brings the tAvo 

 structures closer together. 

 At the time that the con- 

 nection of the caudal 

 pharyngeal complex with 

 the pharynx is broken, the 

 ultimobranchial body is 

 well within the lateral ex- 

 tensions of the thyroid, the 

 more medial portion of the 

 gland, descending more ra- 

 pidly, having ^jassed its 

 level. Further descent of 

 the lateral lobes of the 

 thyroid, extension of their 

 dorsal edges and the growth 

 of the mesoderm within 



