288 BULLETIN OF THE 



drical cords of cells in the glomus ; but in most cases I have been readily 

 able to satisfy myself that this appearance had to do with densely packed 

 blood cells lying in a definite vascular tract. I have also occasionally 

 met with invaginations of the superficial (peritoneal) epithelium of the 

 glomus (page 247) ; but it seems to me, even should it be shown that 

 they give rise in the interior to columns of cells, that this would not be 

 a very serious objection to the view which ascribes to the organ a glo- 

 merular function. In favor of that view, many arguments may be ad- 

 duced : (1) the highly vascular nature of the glomus; (2) its position 

 in an open chamber of the body cavity directly opposite the proncphric 

 nephrostomes ; (3) its serial relations with the mesonephric glomeruli ; 

 (4) its appearance and degeneration synchronously with the pronephros ; 

 and (5) the circumstance that its homologue, wherever found in other 

 classes of Vertebrates, is always in equally close relation with excretory 

 tubules. The last argument seems to me the most weighty, and I am of 

 opinion that a comprehensive comparative study proves beyond question 

 the glomerular nature of the structure. 



In the descriptive part of this paper I have stated that, in satisfactory 

 sections through the blood-vessel which leads from the aorta to the glomus, 

 one could frequently observe that the ramifications within the glomus 

 did not appear to be terminal, but that the vessel seemed to give off 

 a lateral branch to the glomus, while the main trunk continued on to- 

 ward the ventral side of the body. An explanation of this condition has 

 occurred to me, which, if confirmed, will be of considei'able morphological 

 significance, though at present I can merely offer it as a suggestion. As 

 we have already seen, the glomus of Selachii, according to Riickert ('88, 

 pp. 239-242), does not receive a separate blood-vessel directly from the 

 aorta, but a rete mirabile is developed in connection with the segmental 

 vessels described by Paul Mayer. I have not succeeded in tracing 

 the main aortic branch to the ventral side of the larva ; but, as far as it 

 could be followed, the course of the vessel between splanchnopleure and 

 entoderm corresponds perfectly with that of one of the segmental ves- 

 sels described by him. It seems to me quite possible that, in Amphibia, 

 the dorsal portion, which is in communication with the glomus, is the 

 only part of these rudimentary vessels which is retained, and that the 

 remaining portion, having ceased to be of functional importance, fails to 

 develop. 



Having completed my survey of our knowledge of the development of 

 the pronephros in the several classes of Vertebrates, I now turn to a 



