348 FISHES j CHAP. XII 



{. 



system. The former are usually well supplied with sympathetic 

 nerve fibres, and contain ganglion-cells in their substance. 



The primitive origin of these organs is very obscure, and as 

 regards their development there is much diversity of opinion. 

 It seems certain, however, that the cortex and medulla of the 

 higher Vertebrates, including their equivalents in the Elasmo- 

 branchs, have independent origins, and the balance of opinion 

 seems to point to the derivation of the cortex from some portion 

 of the germinal coelomic epithelium, while the medulla is derived 

 from the embryonic nerve cells of the sympathetic ganglia. 



Lymphoid Tissue. — In addition to certain of the ductless 

 glands, and the local or diffused masses of their characteristic 

 tissue already mentioned in connexion with the alimentary canal, 

 lymphoid tissue is often abundantly present in other parts of the 

 body. There is, for example, a mass of this tissue on the heart 

 of the Sturgeon (Acipenser). The anterior enlarged portion of 

 the mesonephros, commonly termed the " head-kidney " of the 

 Teleostomi (Fig. 203, B, C), is almost entirely composed of 

 lymphoid tissue,^ which has replaced, wholly or partially, 

 the proper renal structure ; and from the presence of free red 

 blood-corpuscles and of crystals of oxy-haemoglobin and other 

 derivatives of haemoglobin, it may be inferred that the " head- 

 kidney," in common with the more orthodox blood-glands, per- 

 forms a blood -destroying function.^ On the other hand, the 

 example of the spleen, which is alike the seat of leucocyte- 

 formation and of blood - destruction, renders it unnecessary to 

 reject the view that the " head-kidney " is an organ in which 

 leucocytes or blood-corpuscles are formed. In but few Teleostomi 

 is a purely lymphoid " head-kidney '' entirely wanting, as, for 

 example, in the Sun-Fish {Orthagoriscus mola).^ As previously 

 mentioned the Dipnoi are rem_arkable for the extraordinary 

 development of lymphoid tissue, inasmuch as it forms a thick 

 investing mass round the kidneys and gonads in addition to its 

 exceptional abundance in the walls of the alimentary canal. 



The absence of ordinary lymphatic glands in Fishes is well 

 known, and it is at least probable that, functionally, the want of 

 these lymphoid organs may be compensated for by the super- 

 abundance of lymphoid tissue in other parts of the body.* 



1 Balfour, Quart. J. Micr. Set. xxii. 1882, p. 12. 



2 Swale Vincent, op. cit. p. 78. ^ Ibid. pp. 77, 78. ' Balfour, op. cit. p. 16. 



