BLOOD GLANDS 345 



and Amphioxus, which has undergone a change of function from 

 a mucus-conveying groove to a blood-gland. On the other hand, 

 the mode of origin of the paired thyroids certainly favours the 

 suggestion that they represent a posterior pair of vestigial gill- 

 clefts, a view which derives some support from the fact that in 

 Notidanus, where additional branchial arches and clefts are 

 present, the paired thyroids are absent. 



The Thymus. — In the embryo Elasmobranch and Teleost^ 

 the thymus has a multiple origin, being derived from a series of 

 distinct epithelial thickenings, one of which is developed at the 

 dorsal extremity of each of the gill-clefts except of the spiracle. 

 These rudiments subsequently detach themselves from the epithelial 

 surface and sink inwards, eventually fusing together on each side 

 to form a single independent structure. Later, the epithelial mass 

 thus formed becomes invaded by connective tissue, and by leucocytes 

 which form lymph follicles, and the thymus gradually assumes 

 the structure of a lymphoid organ. From its mode of develop- 

 ment it has been suggested that the thymus owes its evolution 

 to the metamorphosis and ingrowth of branchial filaments,^ but 

 it is also noteworthy that each embryonic rudiment of the organ 

 closely resembles, both in position and origin, one of the develop- 

 ing branchial tongue-bars of Amphioxus.^ The abundance of 

 leucocytes which it contains has also prompted the further 

 suggestion that the origin of the thymus may be due to the 

 necessity of providing for the phagocytic protection of the gills 

 themselves from the ravages of harmful micro-organisms, fungoid 

 spores, etc., as well as to aid in the removal of such portions of 

 the gills as may have been injured.* 



A thymus is probably present in all Fishes, if not in the adult 

 at all events in the embryo, but is always relatively small in size. 

 In Elasmobranchs the organ lies on each side above the branchial 

 arches and beneath the dorsal musculature ; and in Teleostomi at 

 the dorsal extremity of the last branchial arch, in close proximity 

 to the mucous membrane of the branchial cavity. In a similar 

 position in the Dipnoi (e.g. ProtopterusY there are, on each side, 



1 Dohrn, Mitth. Zool. Stat. Neapd. v. 1884, pp. 141-151 ; see also the pre- 

 viously cited works of De Meuron and Maurer. 



^ Dohrn, op. cit. 



2 See pp. 120 and 135. Willey, Amphioxus and the Ancestry of the Vertebrates, 

 New York, 1894, pp. 30, 31. 



* Beard, Anat. Anz. ix. 1894, p. 485. ' ' Newton Parker, op. cit. p. 135. 



