552 ME. JAMES JOHXSTONE ON THE 



and ofhers that the details of that develojjment vary in many 

 important details in different Yertebrata. In the Selachians the 

 thymus originates as a paired series of outgrowths from the 

 dorsal parts of the walls of the 2nd, 3rd, 4th, and 5th branchial 

 pouches. In the ascending series of vertebrates a considerable 

 reduction in the number of gill-sacs involved in the process 

 takes place. In the Batrachians only the 2nd contribute. 

 In Lacerta at least among the reptiles the thymus arises from 

 the 2nd, 3rd, and 4th. In the chick from the 3rd and 4th. 

 But in both lizard and chick it is the part originating from the 

 3rd pouch which is the more important. The dorsal outgrowth of 

 the 4th contributes only a small portion of the definitive organ. 

 In all these classes the thymus takes its origin exclusively from 

 the dorsal portions of the epithelium of the pouches. 



In the Mammalia the reduction goes further. Although 

 the 4th gill-sac does indeed still possibly form a part of the 

 organ, that is only a very small part, de Meuron (5) has 

 worked out the genesis of both thyroid and thymus in the sheep, 

 and I follow his account. In the embryo of 15 mm., the 3rd 

 branchial pouches have become quite separate from the pharynx. 

 The median portion of the pouch (which is indeed the only 

 portion remaining) still shows the original lumen which com- 

 municated with that of the pharynx. On the dorsal side there is 

 a solid swollen thickening of the wall of the pouch, and opposite 

 this, and directed ventrally, is a long diverticulum into which 

 the lumen is prolonged. This ventral diverticulum descends 

 the neck, dragging with it the solid dorsal thickening, and 

 aDproaches the base of the heart. Both parts undergo the 

 same histological changes, and take on the characters of thymus- 

 tissue. They lose their epithelial appearance, and trabeculae 

 of connective tissue invade their substance. The dorsal parts 

 remain for some time attached to the ventral diverticula, which 

 fuse together over the base of the heart by their posterior 

 extremities. 



Similarly the thyroid has a multiple origin. There is the un- 

 paired ventral diverticulum in the middle line, and in addition the 

 4th branchial pouches form paired, ventral, pocket-like prolong- 

 ations, which also enter into the formation of the definitive 

 thyroid body. Like the 3rd branchial pouches, the 4th give rise 

 not only to the ventral pockets but to dorsally placed thickenings. 



