ULTIMOBRANCHIAL BODIES IN POSTNATAL PIGS 21 
area of the left ultimobranchial body is an elongated area, of 
large follicles that extends into the middle third of the thyroid 
gland. <A few very large follicles are found in the immediate 
neighborhood of the left ultimobranchial body in the thyroid 
gland in pig 28 days old. Large follicles are present in the 
immediate neighborhood of the ultimobranchial bodies (?) in 
the young adult hogs nos. 1 and 2. 
In a general way, it can be stated that the unusually large 
follicles are most numerous in the majority of postnatal stages 
in the portion of the thyroid gland in which the ultimobranchial 
bodies are generally found, that is, in the posterior half of the 
gland. ‘This feature is in accord with the fact that in embryos 
from about 50 mm. in length to full term the ultimobranchial 
bodies are also usually located in the posterior half of the 
thyroid gland, although in some they are located in the middle 
third or in the middle two-fourths of the gland. Exceptions to 
the usual location of the majority of large follicles in a single 
gland are not wanting. The. most striking example of this 
exception is found in pig 42 days old, in which the great majority 
of large follicles are found in the anterior half of the thyroid 
gland. In consideration of the variable developmental behavior 
of the ultimobranchial bodies* in pig embryos, it is therefore, 
not surprising (granting an interrelationship between the large 
follicles and the ultimobranchial bodies) to occasionally find 
large follicles out of their usual place (pig 42 days old) or the 
almost entire absence of large follicles, as is the case in the 
thyroid gland in pig 56 days old and in the young adult hog 
no. 3. 
Another significant feature of the location of the very large 
follicles is the depth at which they are embedded in the thyroid 
gland. In the more bulky portion of the gland they are located 
near the dorsal or dorsolateral border of the gland, that is, in 
the region in which the ultimobranchial bodies may be found, 
and not in the ventral or ventrolateral region which is derived 
3 Variations as to their size, the time of their complete transformation into 
typical thyroid structures, their location in the thyroid gland, and the presence 
or absence of large (cystoid) follicles in connection with them. 
