68 WILLIAM MORTON WHEELER, 



In Myxine the capillary nets belonging to these aborted tubules are 

 retained, however, and fuse to form the glomus, which lies behind 

 the pronephros. The nets of the functional pronephric tubules appear 

 to have fused with one another to form a peculiar lymphoid strand, 

 the "strittiges Gewebe" of the late Spengel-Semon controversy. In 

 Petromyzon conditions are much simpler. The glomus lies at the 

 same level as the four or five pronephric tubules and appears to 

 belong to at least three of the segments which originally gave rise to 

 the tubules. There is no "strittiges Gewebe". 



Passing to a consideration of the Gnathostome glomus and 

 glomerulus graver difficulties are encountered. These are not apparent 

 in the Selachians, because they have no distinct glomerular structure 

 associated with the pronephros, but in Ganoidei, Teleostei, Gymno- 

 phiona and Reptilia, a serious difficulty may exist if we accept Felix' 

 view that the glomerulus of these forms is invaginated into the tubule 

 itself, thus bearing the same relations to the pronephric tubule that 

 is borne by the glomerulus to the mesonephric tubule. The views 

 of Felix and Maas may both be true, but on this supposition, we 

 must conclude, either that the retiform glomus of the primitive pro- 

 nephric tubule has become invaginated into the tubule to form a 

 structure indistinguishable from a mesonephric glomerulus instead of 

 emancipating itself from the pronephros and depending freely into 

 the body cavity, or that the socalled pronephric tubules of the bony 

 fishes are not pronephric tubules at all but modified mesonephric 

 tubules ! It is perhaps easier to seize the former horn of the dilemma, 

 but the latter is not so remote as it may appear at first sight. In 

 Felix' minute account (1897 b) of the pro- and mesonephros of the 

 Salmonid several facts suggest that there may be no great fundamental 

 difference between the formation of the pro- and mesonephros. This 

 is also borne out by Felix' general conclusions. The mesonephric 

 tubules are budded off from the pronephric duct, and I have already 

 suggested that in this case we must suppose that the pro- and meso- 

 nephros are cut off from the mesoderm together and afterwards dis- 

 sociated. Why may not a similar explanation apply to the pronephros 

 which Felix describes and figures as arising from both somatopleure 

 and splanchnopleure, an origin incompatible with that of a typical 

 pronephros, which should originate from the former layer only, but 

 quite comprehensible, if we suppose that the pro- and mesonephric 

 material is here, too, combined at the outset. On this supposition a 

 true pronephros sensu stricto would be absent in the Teleost and the 



