ON THE HEAD KIDNEY OF MYXINE. 357 



disappeared, their place being taken by many smaller cells. 

 The whole tissue is richly supplied with blood-vessels pro- 

 vided by the vascular supply to the glomeruli and the 

 capillary network into which this breaks up, and answers 

 with great exactness to the description of the mass of 

 lymphatic tissue in the anterior region of the Ganoid ex- 

 cretory system given by Balfour (1). 



It would seem, therefore, that the head kidney of Myxinoids 

 may be regarded as a stage in the phylogenetic reduction of 

 this organ — a reduction which continues in the Pisces until 

 the tubular structure entirely disappears. 



As regards the relations of this organ with the supra-renal 

 bodies, the absence of nerve-structures would seem at first 

 sight to exclude the possibility of any connection between 

 the two, which, indeed, is the view advanced by Emery (3). 

 A consideration of the subject, however, shows that this is 

 not an insuperable objection. 



In his description of the supra-renals of Elasmobranchs, 

 Balfour^ states that these organs are derived partly from the 

 mesoblast and partly from the sympathetic ganglia, and that 

 in fact the two constituents remain distinct in this group 

 throughout the life of the animal; whilst Mitsukuri has 

 demonstrated the compound origin of these organs in the 

 Mammalia (6). 



It may, therefore, be concluded that the pronephros in 

 Myxine represents the mesoblastic part of the supra-renal 

 bodies, which have been shown by Professor Weldon (10) 

 to be derived from the anterior part of the mesonephron in 

 the higher Vertebrata. 



'b' 



^ ' Comparative Embryology,' vol. ii, p. 664. 



