UROGENITAL SYSTEM OF MYXINOIDS 91 



the midregion of the pronephros. The diameter of the lumen of 

 the duct varies widely in an individual specimen. It may be as 

 small as a tubule in places, then increase to several times that 

 width. The duct and its lumen may be continuous throughout 

 the entire length of the pronephros or may be broken up into dis- 

 connected segments. The most continuous duct was found in a 

 young male. The duct in this specimen also has the least amount 

 of central mass connected with it, and the mass is almost entirely 

 confined to the posterior end of the duct. 



The structure which has been referred to as the 'central mass' 

 was first described by Kirkaldy ('94) for Myxine, and the ^\Titer 

 has adopted his name for it. It is possible that Weldon's lym- 

 phatic tissue may have been the same thing as this central mass, 

 but his short description does not enable us to be positive. This 

 mass of tissue has been the subject of much discussion by investi- 

 gators of the pronephros of Myxinoids. Weldon ('84) described 

 a mass of lymphatic tissue which he found at the posterior end 

 of the head-kidney of Bdellostoma forsteri. He states that the 

 central duct ends posteriorly " in a mass of tissue .... re- 

 sembling the trabecular supporting tissue of a lymphatic gland." 

 Into this mass strands of blood vessels pass from the glomerulus 

 which lay beside it. Upon the basis of this lymphatic tissue 

 Weldon thinks the pronephros becomes transferred into and func- 

 tions as a suprarenal body. Kirkaldy found a difference in the 

 mass, in animals without ova and those with them. In the former 

 the mass was divided into separate parts, and each part was re- 

 garded as a glomerulus supplied with a capsule, inside of which 

 was a characteristic loop (his fig. 2). In an animal with ova he 

 found no central duct or glomerulus in the pronephric vein, but 

 their former position was occupied by a mass of small cells with 

 small nuclei and larger cells with large, round, and deeply stained 

 nuclei (his fig. 7). Capillaries were numerous in this mass. 

 Kirkaldy regards the mass as the degenerating central duct of 

 earher stages, hence Myxinoids may be considered as representing 

 a stage in the phylogenetic reduction of the head-kidney, and the 

 latter may represent the mesoblastic part of the supra-renal 

 bodies. 



