xII 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 epithelal 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, ete., 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. Protopterus)’ there are, on each side, 
1 Dohrn, Mitth. Zool. Stat. Neapel. v. 1884, pp. 141-151; see also the pre- 
viously cited works of De Meuron and Maurer. 
2 Dohrn, op. cit. 
3 See pp. 120 and 135. Willey, Amphiowus and the Ancestry of the Vertebrates, 
New York, 1894, pp. 30, 31. 
4 Beard, Anat. Anz. ix. 1894, p. 485. > Newton Parker, op. cit. p. 135. 
