HEAD KIDNEY OF TELEOSTEAN FISHES. 45 



being posterior. At its broadest or anterior end it suddenly divides,, 

 each branch becoming consttncted and then again dilating to form 

 the large head kidney. The appearance of sections taken at this 

 constricted part in no way differs from that of the body kidney 

 proper, but in the fully-grown adult, the part where the dilatation 

 to the head kidney begins shows an altered condition ; uriniferous 

 tubules have become fewer in number, blood-vessels have disap- 

 peared, and the cells of the surrounding matrix seem to have multi- 

 plied. Still further forward, hardly any trace of tubules remains. 

 This condition is shown in fig. 2. 



Here, elongated empty spaces alone denote the former position 

 of tubules, and the granular matrix forms almost the entire organ ; 

 whereas in sections of the body kidney, the convoluted tubules 

 lie so thickly together as to leave room for little surrounding 

 substance. As to the nature of this matrix, which Balfour considers 

 to be lymph, I am not prepared to make a positive statement. 

 Looked at with a very high power, the granules are seen to be 

 nucleated, and to possess a more or less irregular outline, but they 

 are extremely small, and it seems to me the entire organ presents a 

 singularly solid mass to be analagous to a lymph gland. Capillaries 

 do not appear to exist, nor can I find any adenoid reticulum. 



The very early condition of the pronephros is seen in fig. 3 — a 

 thirteen days^ embryo. Here the tubules are still few in number, but 

 large in proportion to the size of the organ, and there is a consider- 

 able mass of granular tissue. As development proceeds the tubules 

 multiply, and there is consequently less granular substance, but a 

 Cyclopterus three quarters of an inch long shows little difference in 

 the condition of its pronephros, from the figure of the thirteenth day 

 embryo. 



I have preserved almost a complete series of specimens from the 

 fertilization of the egg onwards, but find that not till Cyclopterus 

 has become sexually mature does its head kidney commence to dege- 

 nerate. Fig-. 4 shows a section of the pronephros of what may 

 be described as a half grown or small adult fish, but owing to the 

 specimen having been preserved in strong spirit, great shrinkage 

 has taken place. This figure is in strong contrast to the condition 

 in the old fish as seen in fig. 2, yet it is very similar to the figure 

 showing the old state in Dactylopterus, fig. 1. 



Returning now to the statement of Balfour that, in some instances, 

 the mesonephros grows forward so as to take up the position 

 formerly occupied by the pronephros, and again considering the case 

 of Dactylopterus, two objections suggest themselves. First, in the 

 developing embryo, the segmental duct and pronephros are deve- 

 loped at a much earlier period than the mesonephros, and must be 



