PLATE 1 



EXPLANATION OF FIGURES 



1 From a photogi'aph of a transverse section of the ultimobranchial bodies 

 and the thyroid gland about midway between the anterior and posterior ends of 

 the latter. Before the fusion of the ultimobranchial bodies with the thyroid 

 gland. From an embryo 18 mm. long. X 60. 



2 a and 2 b From photographs of transverse sections of the ultimobranchial 

 bodies and the thyroid gland taken respectively near the anterior and posterior 

 ends of the latter. P'usion between the ultimobranchial bodies and the thyroid 

 gland has in some places taken place. From an embryo 19.5 mm. long. X 60. 



3 a, 3 b, and 3 c From photographs of transverse sections of the ultimobran- 

 chial bodies and the thyroid gland taken respectively near the anterior, middle, 

 and posterior portions of the latter. The numerous deeply stained nuclei in the 

 ultimobranchial bodies are represented by small black dots. From an embryo 20 

 mm. long. X 60. 



4 From a photograph of a transverse section about midway between the 

 two ends of the tripartite complex. The horns of the crescent are largely composed 

 of the ultimobranchial bodies the left one of which is quite irregular along its 

 dorso-mesial surface due to epithelial buds. From an embryo 21 mm. long, 

 X 60. 



5 From a photograph of a transverse section about mid.vay between the two 

 ends of the tripartite complex showing the large size of the ultimobranchial 

 bodies. From an embryo 21.5 mm. long. X 60. 



6 a and 6 b From photographs of transverse sections through near the middle 

 and caudal portions respectively of the tripartite complex, showing nvunerous 

 deeply stained nuclei (small black dots in the figures) in the ultimobranchial 

 bodies and a few in the cell cords of the thyroid gland. The figures also show 

 that the caudal portion of the ultimobranchial bodies are less broken up into cell 

 cords than their more anterior portion. From an embryo 22 mm. long. X 60. 



7 From a photograph of a portion of an ultimobranchial body showing degen- 

 erated and deeply stained nuclei. From an embryo 23 mm. long. X 650. 



D.N., degenerated nuclei T., thyroid gland 



D.S.N. , deeply stained nuclei Tr., trachea 



Ep. B., epithelial buds U., ultimobranchial body 

 L., lumen 



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