Till 



I 



in;.) a spindle-flhaped organ ! 



a hove tin- in>ertion of t lie gill ai < 

 which subsequently becomes inde- 

 pendent, just as it dor>in Selachians. 

 The originally epithelial product ac- 

 quires a peculiar hi.Mologieal char- 

 acter from being penetrated by 

 ingrowths of connective tissue ele- 

 ments. In the first place Ivmnh- 

 cells in great quantities migrate in 

 between the epithelial cells, in a 

 manner >imilar to that described by 

 STOIIK as of frequent occurrence in 

 the territory of mucous membranes. 

 Secondly, the epithelial growth is 

 ira\er-ed in all directions and cut 

 up into small port ions by connective 

 tissue, in which lymph-follicles are 

 formed. The thymus thereby ac- 

 quires the appearance of a lymplioid 

 organ, in which the epithelial rem- 

 nants are still in par; preserved, 

 but only in the form of very small 

 spherical portions, as the cci-pn 

 of II ASSALL. At a still later stage 

 of development there arise in the 

 organ irregular cavities tilled with 

 molecular granules. These are 

 d by the disintegration of 

 lymph-colls and tin? melting d .wn 

 of the reticular connective tissue, 

 which takes place here and tl 



In the higher, nir-bivat hing Ver- 

 tehrates the thymus i- derived either 

 from the epithelium of two or three 

 clefts or only from the epithelium 

 of the third visceral clett, which 

 Uvomes closed. The former is tin- 

 case with Reptiles (tig. 170 A t/i) 

 and Ulnls (jig. 17b /> tl,}. the latter 

 with Mammals. In Reptiles and 



Fig. 177. Semi diagrammatic illustra- 

 tions to show the ultimate position of 

 thymus, thyroid gland, and accessory 

 thyroid gland on the neck of the 

 Lizard (.4), the Chick (B), and the 

 Calf (O, after DE MEUEON. 



s-t, Thyroid gland ; ntd, accessory thyroid 

 i; th, thjmus; (A 1 , accessory 

 tliy::. . heart ; vj 



vena jugularis; ca, carotid v 



