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 1 . Thymvs. 



The developineiit of the tlivinns in the Selachians was first 

 described by Dohkn (1884). The facts then found by him were 

 principally contirnied by later investigators. Hammak, who had given 

 many years to the stndy of the structure, development and function 

 of this organ in nearly all the principal groups of vertebrates, 

 described the development in the Selachians in 1912, and gave a 

 detailed account of the results of his predecessors. 



He found, that in all vertebrates from ilsh up to man, the 

 thymus continues to grow till the time of puberty. Then an involu- 

 tion period begins, wherein it as a rule atiopliies, without totally 

 disappearing. 



The thymus, in all vertebrates, begins to form as a local pro- 

 liferation of the epithelium of the gill clefts. 



In man it appears principally, if not exclusively, on the third 

 gill cleft, but in the Selachians, which generally have six gill clefts, 

 a beginning of the thymus is described on each gill cleft. These 

 however speedily disappear on the first and last, sometimes even on 

 the last two gill clefts. 



Not all investigators are of opinion tiiat the thickening of epithe- 

 lium cells of the first gill cleft (spiracle) may l)e considered as a 

 thymus, and it is possible that here an interchange may have taken 

 place with the place of origin of the spiracular sense organ. 



Soon after its appearance, one can distinguish in the thymus two 

 difterent kinds of cells, viz,, a network of flat epithelial cells, which 

 encloses groups of round cells in its meshes. 



These round cells multi[)ly themselves so quickly, that the net- 

 work can no longer be discerned uidess in very thin sections. 



The whole organ, which formerly was pear-shaped and after- 

 wards has the shape of a grape bunch, appears to be wholly 

 constituted of round cells, which form a solid mass without lumen. 

 These cells hardly have any protoplasm, and therefore give the 

 appearance as if one only has to do with an accumulation of nuclei. 



There are two opinions concerning the derivation of these round 

 cells, which strongly resemble the lymphocytes of the blood. Many 

 hold them for epithelium cells, which have rounded theujselves off; 

 others again take them to be true lymphocytes, which have 

 penetrated the organ from the bloodvessels and the neighbouring 

 mesenchym. The latter opinion is emphatically upheld by Hammah for 

 all classes of vertebrates. 



The question as to which of the two opinions is correct, cannot 



