ULTIMOBRANCHIAL BODIES IN POSTNATAL PIGS 17 
Pig 42 days old (fig. 3). The thyroid gland is 14.6 mm. long 
and its greatest width is 7.3 mm. It tapers to a point at each 
end and, excepting near its ends, is crescent in shape in trans- 
verse section. The ultimobranchial body on the right side is 
located midway between the two ends of the thyroid gland and is 
embedded just below the dorsal surface of the gland lateral to 
its medial plane. It extends through a series of twenty-six 
sections and is composed of syncytial cords and masses. In 
both ends and in the peripheral portion of this structure the 
follicles are quite numerous but comparatively small, while in 
places its center is free from colloid. The ultimobranchial body 
on the left side is located in the anterior portion of the caudal 
fourth of the thyroid gland and is embedded just below the dorsal 
surface of the gland lateral to its medial plane. It extends 
through fifty-four consecutive sections and has a structure 
(fig. 3, U.) similar to the right one. In place it is almost 
separated from the rest of the thyroid gland by connective 
tissue. 
An area of large follicles in the right lateral half of the thyroid 
gland extends anteriorly from the ultimobranchial body. This 
area of large follicles is located chiefly just below the dorsal 
border of the gland, but in places it extends to its dorsalateral 
margin. In the dorsolateral portion of the middle third of the 
left lateral half of the thyroid gland is an area, variable in width, 
of very large (cystoid) follicles. Near the caudal portion of the 
anterior third of the thyroid gland these areas of large follicles 
become continuous just below its dorsal surface and gradually 
extend more deeply into the gland so that large follicles are 
found throughout its extreme anterior portion. No follicles of 
an unusually large size are found in the extreme caudal portion 
of the thyroid gland. 
Pig 56 days old. The thyroid gland is 13.5 mm. fae and its 
greatest width is 10.2 mm. It tapers to a blunt point at both 
ends. The left lateral half of the gland is considerably more 
bulky than the right lateral half. The only traces of an ulti- 
mobranchial body is an area of very small follicles near the 
dorsolateral margin in the right lateral half of the thyroid gland. 
