82 
The pronephros degenerates at a variable age. We 
have seen it in a plaice of about 4 inch in length, that is 
at an age when the metamorphosis of the larva is com- 
plete and the adult asymmetry thoroughly established. 
The Head Kidney.—In a continuous series of sections 
from a plaice of about one inch long passing through the 
whole length of the kidney, the latter organ can be seen 
to be divided into three ill-defined parts. The posterior 
or thickened portion of the kidney is crowded with 
uriniferous tubules cut in various planes, Malpighian 
corpuscles can be seen, though these are very few, and the 
lymphatic tissue is (relatively) not abundant. Anterior to 
this, and occupying the thinnest middle portion of the 
kidney, is a region where the segmental duct, slightly 
expanded, alone persists, but no uriniferous tubules are 
present in the sections. his is the intermediate portion 
of the kidney. Anterior to this, and beginning at the 
plane of transition of stomach into cesophagus, is a region 
where the segmental duct becomes thrown into convolu- 
tions. Here, too, the kidney divides into the two anterior 
horns which lie on either side of the cesophagus. This is 
the head kidney, and it contains lymphatic tissue. This 
tissue is present through all the length of the kidney, but 
is more abundant in the anterior portion, and here it 
is aggregated into nodules with well-marked blood chan- 
nels between : that is, it has characters intermediate be- 
tween a true lymphatic and haemolymph gland. 
In the oldest specimens investigated this swollen 
anterior portion of the kidney has no traces of uriniferous 
tubules or segmental duct. It consists only of a modified 
form of lymphatic tissue with large blood vessels. In it 
are nests.of black pigment in the form of irregular 
granules, and its wall also is deeply pigmented. 
These anterior swollen portions are the degenerate 
