FATE OF THE ULTIMOBRANCHIAL BODIES 93 



length only the neck, from which the sides and the cervical ver- 

 tebrae were removed, was reserved. From embryos 100 to 270 

 mm. in length (full term) only the thyroid with some of the sur- 

 rounding structures — trachea, esophagus, a portion of the thy- 

 mus, etc., was removed. The length in millimeters of the differ- 

 ent developmental stages of which the thyroid was prepared for 

 a study of the ultimobranchial bodies is as follows: 15, 16, 17, 17, 

 18, 18, 19, 19.5, 20, 20, 21, 21, 21.5, 22, 22, 23, 23, 23, 24, 24.5, 25, 

 25, 26, 27, 27, 28, 29.5, 30, 33, 35, 37.5, 38, 40, 48, 53, 60, 65, 

 65, 75, 100, 100, 111, 125, 125, 145, 150, 160, 175, 225, 245, 270, 

 and 270. These figures represent the length of the embryos while 

 in a fresh condition. 



The fixing fluids employed were Zenker's fluid, Zenker-formol, 

 and Picro-aceto-formol. The materia) was imbedded in paraf- 

 fi.n. The earlier embryos were cut transversely in sections 5 

 microns thick, while those of later stages in sections 8 to 10 

 microns thick. Various stains were used. For embryos 15 to 

 65 mm. in length, iron hematoxylin gave the best results. The 

 thyroid gland of later developmental stages was stained with 

 Chloral hematoxylin and eosin, and eosin-methylene blue. 



IV. DESCRIPTION OF STAGES 



The earliest stage chosen for description is one just before the 

 ultimobranchial bodies have fused with the thyroid gland. 'These 

 structures will be described in two embryos of the same size 

 only when there is a marked contrast in their size, structure, or 

 position in the thyroid in the two embryos. 



Embryo of 18 mm. (fig. 1). The ultimobranchial bodies have 

 lost their connection with the fourth (?) pharyngeal pouch and 

 extend cephalad beyond the anteror margin of the thyroid gland. 

 Their anterior portion is in form a slender tube, ovaF in cross 

 section, and with wall two to three layers of cells (nuclei) thick. 

 Caudalward the walls of these structures gradually becomes 

 thicker. In the portion in relation to the thyroid gland the 

 lumen in places is obliterated and the remnants persist as mere 

 slits. Anteriorly the surface of these tubules is quite smooth, 

 while caudal] y irregularities occur on their surface. Their cau- 



