THE REGION OF THE SPINAL CORD 419 



to excrete to the exterior ? What should we look for in our search 

 after the lost coxal glands ? 



The answer to these questions is most plainly given in the case 

 of the pronephros, especially in Myxine, where Maas has been able 

 to follow out the whole process of the conversion of nephric tubules 

 into a tissue resembling that of a lymph-gland. 



He states, in the first place, that the pronephros possesses a 

 capillary network, which extends over the pronephric duct, while 

 the tubules of the mesonephros possess not only this capillary net- 

 work, equivalent to the capillaries over the convoluted tubules in the 

 higher vertebrates, but also a true glomerulus, in that the nephric 

 segmental arteriole forms a coil (Knauel), and pushes in the wall of 

 the mesonephric tubule. He describes the pronephros of large adult 

 individuals as consisting of — 



1. Tubules with funnels which open into the pericardial ccelom. 



2. A large capillary network (the glomus) at the distal end. 



3. A peculiar tissue (the ' strittige Gewebe ' of the Semon-Spengel 

 controversy), which Spengel considers to be composed of the altered 

 epithelium of pronephric tubules, while Semon looks on it as an 

 amalgamation of glomeruli. 



Maas is entirely on the side of Spengel, and shows that this 

 peculiar tissue is actually formed by modified pronephric tubules, 

 which become more and more lymphatic in character. 



He says : " The pronephros consists of a number of nephric 

 tubules, placed separately one behind the other, which were origi- 

 nally segmental in character, each one of which is supplied by a 

 capillary network from a segmental branch of the aorta. The 

 tubules begin with many mouths (dorso-lateral and medial- ventral) 

 in the pericardial cavity ; on their other blind end they have lost 

 their original external opening, and there, in the cranial portion of 

 the head-kidney, before they have joined together to form a collecting 

 duct, they, together with the vascular network, are transformed into 

 a peculiar adrenal-like tissue. The most posterior of the segmental 

 capillary nets retain their original character, and are concentrated 

 into the separate capillary mass known as the glomus." 



Later on he says : " Further, the separate head-kidney is more and 

 more removed in structure from an excretory organ in the ordinary 

 sense. One cannot, however, speak of it as an organ becoming rudi- 

 mentary ; this is proved not only by the progressive transformation 



