DEVELOPMENT OF THIRD DAY 



497 



must already at this early period pass directly into and through the liver. 

 If this is remembered, it will make the adult portal circulation the better 

 understood. 



THE PANCREAS 



The pancreas arises as three diverticula from the duodenum at the 

 approximate level of the liver diverticulum. There are three pancreatic 

 buds, one medial, lying dorsal to the duodenum, and a pair of ventro- 

 lateral buds. 



The median bud appears at about seventy-two hours, while the two 

 ventro-lateral buds can be seen at the end of the fourth day. The 

 dorsal bud arises directly opposite the liver, and grows into the dorsal 

 mesentery, while the ventro-lateral buds arise at the point where the 

 liver connects with the intestine, so that both the liver duct and the 

 ventral pancreatic duct open into the duodenum by a common duct 

 called the ductus choledochus. Cellular cords grow into masses from the 

 three buds, fusing into one glandular mass with two ducts remaining, 

 although sometimes all three remain. 



THE THYROID GLAND (Fig. 294) 



This arises as a median diverticulum from the floor of the pharynx 

 at the level of the second pair of gill pouches. By the close of the fourth 

 day, the solid, rod-like diverticulum lying in a longitudinal position 

 under the floor of the pharynx has become saccular, remaining connected 

 with the point of origin as the thyro-glossal duct opening at the root 

 of the tongue. In mammals, there are additional evaginations at the lat- 

 eral region of the fourth gill pouch. By the sixth day, the thyroid body 

 in the chick becomes bi-lobed, the lobes sending out cords of tissue 

 which become hollowed out to form the regular adult thyroid tissue. 



The gland then shifts backward and 

 becomes surrounded with a sheath 

 of vascular connective tissue. 



THE THYMUS GLAND 

 (Fig. 294) 



This organ arises from the pos- 

 terior faces of the third and fourth 

 gill pouches after the fourth day of 

 incubation. While the organ is orig- 

 inally epithelial in character, there is 

 soon an ingrowth of mesenchyme 

 and the thymus then becomes chiefly 

 lymphoid in structure. 



Fig. 294. 



Diagrams to show the development of the 

 derivatives of the digestive tract in the 

 branchial region. A, Anura, B, lizard, cd, 

 carotid gland ; e 1 -e a , epithelial bodies ; Krd. 

 Krm, Krv, dorsal, mid, and ventral remains of 

 the axial portions of the gill pouches ; p. 

 postbranchial bodies ; Tml=3, Thymus anlage ; 

 Tr, Thyroid gland; I-V, Gill slits (After 

 Maurer. ) 



