66 WILLIAM MORTON WHEELER, 



Such a fusion appears to have taken place in Petromyson, in which 

 the multiple arterioles point to the former existence of at least three 

 segmental glomi. In the case of the mesonephros only glomeruli are 

 developed and these therefore correspond to only one of the three 

 modes recognized in the pronephros, viz. to the formation of a chamber 

 by dilatation of the lumen of a tubule and the protrusion into it of 

 the capillary tuft. Hence the pronephros may exhibit either true 

 glomeruli, or glomi, the mesonephros glomeruli only. 



While Felix' observation and criticism have been decidedly de- 

 structive of certain false generalizations, they can hardly be said to 

 have any very great constructive value. They leave us without an 

 explanation, as he himself says (1897a, p. 596): "In welchen Mo- 

 menten die Ursache für die Bildung des Innern oder des äussern 

 Glomerulus zu suchen ist, ist mit Bestimmtheit wohl niemals anzu- 

 geben." But unexpected and brilliant light has been shed over the 

 whole subject of the glomus and glomerulus by Maas' study of the 

 young Myxine. It comes as a relief after the fruitless controversy 

 of Semon, Spengel and Felix, who based their diverse interpretations 

 on the anatomy of the adult Myxine and Price's fragmentary researches 

 on the embryos of Bdellostoma. Maas' researches furnish material 

 for an entirely new conception of the glomerulus. The antithesis 

 between glomus and glomerulus, strongly suspected by Van Wijhe 

 and others, and expressed in the names which he applied to these 

 structures, may prove to be a true morphological distinction. Ac- 

 cording to Maas the glomus is the homologue of one or more of the 

 capillary nets found by Boveri investing each nephridium in Am- 

 phioxus. These capillary nets are supplied by metameric arterioles 

 from the aorta. In the Craniote they return their blood to the car- 

 dinal veins. Since , as Maas has shown , there is every reason to 

 suppose that in Myxine the pronephric duct is the modified pro- 

 nephros of the trunk region behind the pronephros sensu stricto, the 

 net-work of capillaries investing the glandular pronephric duct is 

 serially homologous with the metameric capillary nets belonging to 

 the pronephric tubules. Even in the trunk region Maas has recog- 

 nized traces of a metameric arrangement of the net-work enveloping 

 the duct. The mesonephric tubules, however, have acquired a second 

 series of vascular structures of a more specialized form, the metameric 

 glomeruli, which are intercalated in the course of the nephric arterioles 

 before these vessels spread out to form the capillary nets. The 

 glomus of the typical Craniote pronephros is interpreted by Maas in 



