ULTIMOBRANCHIAL BODIES IN POSTNATAL PIGS 19 
point. No remnants of the ultimobranchial bodies are present. 
Only a few follicles of an unusually large size are present in the 
extreme anterior and posterior ends of the gland. 
SUMMARY 
In pig embryos? it was found that the peripheral portion of 
the ultimobranchial bodies generally develops into typical thyroid 
structures before its more central portion. It thus becomes 
evident that the ultimobranchial bodies which can be recognized 
structurally as such in the thyroid gland of postnatal pigs are 
mere remnants of these structures that have not fully developed 
into typical thyroid structures. Also, the structure of the 
ultimobranchial bodies in the late developmental stages of pig 
embryos and in the postnatal pigs are similar, namely, areas 
composed of nucleated syncytial cords and masses the central 
portion of which may be free frum colloids. Areas of small 
follicles (developmentally young) are not so marked in the 
immediate neighborhood of the ultimobranchial bodies in post- 
nata pigs as in the immediate vicinity of these structures in 
most of the late embryonic developmental stages. 
One of the ultimobranchial bodies in the thyroid gland of 
three pigs is not a continuous structure, but is broken up into 
segments. For example, the one on the left side in the 7.5-day- 
old pig is composed of three oblong areas which vary in length. 
It seems that a satisfactory explanation for this condition is a 
more rapid transformation into typical thyroid structures of 
some parts of the more central portion or core than of other 
parts, thus dividing it into segments which are separated from 
each other by follicles. The length of the portion of an ultimo- 
branchial body broken up into segments is obtained by measur- 
ing the distance between the anterior and posterior parts of the 
most anteriorly and most posteriorly located segments, re- 
spectively. Thus the left ultimobranchial body in the 7.5-day- 
old pig extends through a series of seventy-five sections. The 
2 In the summary it will be necessary to refer quite frequently to the writer’s 
previous work on the ultimobranchial bodies in pig embryos. This will be done 
without calling attention to the bibliographic reference of that article. 
