82 



The pronephros deg"enerates at a variable age. We 

 have seen it in a plaice of about i inch in length, that is 

 at an age when the metamorphosis of the larva is com- 

 plete and the adnlt 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 

 uriniferoiis 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. This is the intermediate portion 

 of the kidney. Anterior to this, and beginning at the 

 plane of transition of stomach into oesophagus, 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 oesophagus. 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 



