. The development of the chick; an introduction to embryology . The second visceral pouch leaves noderivatives in the adult; during the fourth day, however, a con-siderable thickening of the epithelium appears on its dorsal andposterior aspect, near its opening into the pharynx; though thisdisappears very soon, it may be considered to represent thethynuis II of Selachia and Anura. The third visceral pouch loses its connection with the pharynxby atrophy of its internal portion between the seventh and eighth 308 THE DKVFXOPMENT OF THE CHICK tUiys, and its intermediate portion persists as an epit
. The development of the chick; an introduction to embryology . The second visceral pouch leaves noderivatives in the adult; during the fourth day, however, a con-siderable thickening of the epithelium appears on its dorsal andposterior aspect, near its opening into the pharynx; though thisdisappears very soon, it may be considered to represent thethynuis II of Selachia and Anura. The third visceral pouch loses its connection with the pharynxby atrophy of its internal portion between the seventh and eighth 308 THE DKVFXOPMENT OF THE CHICK tUiys, and its intermediate portion persists as an epithelial pocketon the ventral face of the jugular vein (Fig. 178). This pocketsoon divides into dorsal and ventral moities of which the formerdevelops into the chief part of the thymus (thymus III) and thelatter into the so-called epithelial vestige III. (See below.) The fourth visceral pouch likewise separates from the pharynxon the seventh day, and furnishes from its dorsal portion thethymus IV, and from its ventral portion epithelial vestige IV.(See below.). Fig. 178. — The derivatives of the embryonic of the chick. (AfterVerdun from Maurer.) A. Of 7 days inciiliatioii. B. Of 8 days incubation. Ep. 3, Ep. 4, Ej)itheHal l)0(U(^s derived from the tliird and fourth visceralpouches. J., .bigular vein. j)br (V)., Postbranchial bodies derived fromthe fifth visceral j)oucli. Ph., Pharynx. Th. .3, Th. 4, Parts of the thymusderived from the third and fourth visceral pouches respectively. TV., Thy-roid. Ill, IV, third and fourth visceral clefts. The fifth ])()uch (postbranchial body) likewise liecomes iso-lated on the seventh day as a closed vesicle: its diffcMentiation isconsidered below. The Thymus. .Vccoi-ding to llu^ abo\e. the thymus of thechick has a douljle origin on each side; \\\c main porlion (thymusIII) is derived from the dorsal wall of the intermediate ]iart ofthe third \isceral potich. This soon elongates to form an epi-thelial cord extending along the jugular \
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