THE ORGANS OF THE INNER GERM-LAYER. 



315 



into a spindle-shaped organ lying 

 above the insertion of the gill-arches, 

 which subsequently becomes inde- 

 pendent, just as it does in Selachians. 

 The originally epithelial product ac- 

 quires a peculiar histological char- 

 acter from being penetrated by 

 ingrowths of connective-tissue ele- 

 ments. In the first place lymph- 

 cells in great quantities migrate in 

 between the epithelial cells, in a 

 manner similar to that described by 

 STOHR as of frequent occurrence in 

 the territory of mucous membranes. 

 Secondly, the epithelial growth is 

 traversed in all directions and cut 

 up into small portions by connective 

 tissue, in which lymph-follicles are 

 formed. The thymus thereby ac- 

 quires the appearance of a lymphoid 

 organ, in which the epithelial rem- 

 nants are still in part preserved, 

 but only in the form of very small 

 spherical portions, as the corpuscles 

 of HASSALL. At a still later stage 

 of development there arise in the 

 organ irregular cavities filled with 

 molecular granules. These are 

 caused by the disintegration of 

 lymph-cells and the melting down 

 of the reticular connective tissue, 

 which takes place here and there. 

 In the higher, air-breathing Ver- 

 tebrates the thymus is derived either 

 from the epithelium of two or three 

 clefts or only from the epithelium 

 of the third visceral cleft, which 

 becomes closed. The former is the 

 case with Reptiles (fig. 176 A th) 

 and Birds (fig. 176 B th}, the latter 

 with Mammals. In Reptiles and 



**-- 



"Fig. 177. Semidiagrammatic illustra- 

 tions to show the ultimate position of 

 thymus, thyroid gland, and accessory 

 thyroid gland on the neck of the 

 lizard (A), the Chick (B), and the 

 Calf (C), after DE MEUBON. 



d, Thyroid gland ; mil, accessory thyroid 

 gland ; th, thymus ; th 1 , accessory 

 thymus ; ir, trachea ; h, heart ; vj 

 vena jugularis ; ca, carotid vein. 



