308 THE DEVELOPMENT OF THE CHICK 
days, and its intermediate portion persists as an epithelial pocket 
on the ventral face of the jugular vein (Fig. 178). This pocket 
soon divides into dorsal and ventral moities of which the former 
develops into the chief part of the thymus (thymus III) and the 
latter into the so-called epithelial vestige III. (See below.) 
The fourth visceral pouch likewise separates from the pharynx 
on the seventh day, and furnishes from its dorsal portion the 
thymus IV, and from its ventral portion epithelial vestige IV. 
(See below.) 
Fria. 178. — The derivatives of the embryonic pharynx of the chick. (After 
Verdun from Maurer.) 
A. Of 7 days’ incubation. 
B. Of 8 days’ incubation. 
Ep. 3, Ep. 4, Epithelial bodies derived from the third and fourth visceral 
pouches. J., Jugular vein. p’br (V)., Postbranchial bodies derived from 
the fifth visceral pouch. Ph., Pharynx. Th. 38, Th. 4, Parts of the thymus 
derived from the third and fourth visceral pouches respectively. T’r., Thy- 
roid. III, [V, third and fourth visceral clefts. 
The fifth pouch (postbranchial body) likewise becomes iso- 
lated on the seventh day as a closed vesicle; its differentiation is 
considered below. 
The Thymus. According to the above, the thymus of the 
chick has a double origin on each side; the main portion (thymus 
III) is derived from the dorsal wall of the intermediate part of 
the third visceral pouch. This soon elongates to form an epi- 
thelial cord extending along the jugular vein; a smaller portion 
(thymus IV) of the thymus is derived from a corresponding part 
of the fourth visceral pouch, and fuses with thymus IIT (Fig. 178). 
