16 r. M. BALFOUR. 



case the burden of proof rests with those who maintain that 

 such remnants are to be found. 



The general result of my investigations is thus to render it 

 prohahle t/iat the pronepliros, thotiglt found in the larva or em- 

 bryos of almost all the Ichthyopsida, except the Elasmobranchii, 

 is always a purely larval organ, lohich never constitutes an active 

 part of the excretory system in the adult state. 



This conclusion appears to me to add probability to the view 

 of Gegenbaur that the pronephros is the primitive excretory 

 gland of the Chordata ; and that the mesonephros or Wolffian 

 body, by which it is replaced in existing Ichthyopsida, is pbylo- 

 genetically a more recent organ. 



In the preceding pages I have had frequent occasion to allude 

 to the lymphatic tissue which has been usually mistaken for 

 part of the excretory organ. This tissue is formed of trabecular 

 work, like that of lymphatic glands, in the meshes of which an 

 immense number of cells are placed, which may fairly be com- 

 pared with the similarly placed cells of lymphatic glands. In 

 the Sturgeon a considerable number of cells are found with 

 peculiar granular nuclei, which are not found in the Teleostei. 

 In both groups, but especially in the Teleostei, the tissue is 

 highly vascular, and is penetrated throughout by a regular 

 plexus of very large capillaries, which appear to have distinct 

 walls, and which pour their blood into the posterior cardinal 

 vein as it passes through the organ. The relation of this tissue 

 to the lymphatic system I have not made out. 



The function of the tissue is far from clear. Its great 

 abundance, highly vascular character, and presence before the 

 atrophy of the pronephros, appear to ]ne to show that it cannot 

 be merely the non-absorbed remnant of the latter organ. From 

 its size and vascularity it probably has an important function ; 

 and from its structure this most either be the formation of lymph 

 corpuscles or of blood corpuscles. 



In structure it most resembles a lymphatic gland, though, 

 till it has been shown to have some relation to the lymphatic 

 system, this can go for very little. 



On the whole, I am provisionally inclined to regard it as a 

 form of lymphatic gland, these bodies being not otherwise 

 represented in fishes. 



