356 J. W. KIRKALDY. 



vessels enter the tissue and break up into fine capillaries, 

 which are caught in section in every direction. The blood- 

 capillaries surrounding the tubes are still distinct, and so is 

 the pericardial epithelium encircling the outermost parts; but 

 that connected with the inner regions, as also the vascular 

 wall, where it is much involved in the branching tubes, is 

 considerably altered. Where at all recognisable the pericardial 

 epithelium has lost its fiat delicate appearance, and has 

 become thickened and heavy-looking : the wall of the vein 

 is also considerably thickened and much broken. 



In sections of head kidneys of any stage 1 find a small 

 quantity of lymphatic tissue (4, 8) lying external to the wall 

 of the post-cardinal vein; but though in the later stages it 

 comes into close relation posteriorly with this ** central mass,^' 

 I do not think the two are connected in origin. The " central 

 mass " would appear clearly to be derived from the breaking 

 down of the central duct of earlier stages. 



Grosglik has already pointed out (4) that the recorded 

 observations on the pronephros of Myxine (8) and Bdel- 

 lostoma (9) were probably made on developing, and not on 

 mature specimens. A study of my sections confirms this 

 view, and also the opinion that the Myxinoid head kidney is 

 undergoing reduction. 



In the younger stages the organ consists of numerous 

 tubules opening at one extremity into the pericardium by 

 means of funnels, and at the other into a main duct, in 

 connection with which are glomeruli. 



As the animal reaches maturity a change takes place, 

 beginning at the hinder end of the organ and working for- 

 wards, afi'ecting first the central part and the ends of the 

 tubules adjacent. The change consists in the formation of 

 a mass of tissue much resembling lymphatic tissue, which 

 would appear to result from the breaking down of the walls 

 of the ducts, the constituent cells furnishing the fibrous 

 groundwork of the new tissue, and also cells with large 

 nuclei, which I am inclined to regard as formative cells, 

 since in the most reduced organs I have examined they have 



