THYMUS-LIKE STRUCTURES IN AMMOCOETES 129 



The bearing on the interpretation of thymus histogenesis in 

 higher animals suggested by the development of the organ in a 

 primitive type, led the author to undertake a systematic study of 

 the branchial region of, the petromyzon larva. The time and 

 work which have been given to this study have, I believe, been 

 amply repaid in the results obtained. Thymus-like placodes 

 have not only been located in the position which makes them 

 homologous with the thymus placodes of the elasmobranchs, 

 but the placodes have also been found in a more primitive con- 

 dition than they have been shown to exist in any other animal. 



LITERATURE 



The search for a thymus in the most primitive chordate ani- 

 mals has been undertaken by a number of investigators Up to 

 the present time the organ has not been established in any of 

 these lower forms Willey ('94) suggests that the tongue-bars 

 occurring in the gill-slits of amphioxus represents the thymus 

 gland. The position of these structures is apparently the only 

 basis for this suggestion Their gelatinous structure, however, 

 would offset any argument that they were homologous with the 

 thymus placodes of fishes. Stannius ('84) credits the discovery 

 of the thymus in the myxinoids to Johannes Miiller. Later 

 investigators, however, have been unable to verify this dis- 

 covery Stockard ('07) in his study of the thyreoid in Bdello- 

 stoma Stouti was unable to find a thymus in this form M. 

 Schultze ('56) described a tortuous sac in the ventral wall of 

 the branchial cavity of Petromyzon planeri which he thought 

 represented a thymus. Schneider ('79) showed that a part of 

 this structure disappears in the development of the animal while 

 the remaining part changes into a group of follicles which repre- 

 sent the thyreoid. 



Schaffer ('94) described structures in the lateral branchial 

 wall of a 51 mm. larva of Petromyzon planeri, which he thought 

 represented thymus anlagen. He found in all twenty-eight 

 anlagen, seven pairs on each side which consisted of ventral and 

 dorsal portions. These anlagen were connected mth the epithe- 

 lium of the branchial vestibules. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 22, NO. 1 



