348 FISHES 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 remarkable 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. Sct. xxii. 1882, p. 12. 
2 Swale Vincent, op. cit. p. 78.  % Ibid. pp. 77,78. 4 Balfour, op. cit. p. 16. 
