no FROM FISH TO PHILOSOPHER 



water fishes, the proximal tubule is divisible into two 

 parts which can be distinguished by the size and struc- 

 ture of the cells. In the aglomerular nephron, the first 

 part, immediately following the glomerulus, has disap- 

 peared, leaving only the second part, so that the tubule 

 in the aglomerular fish is reduced to a single segment, 

 homogenous in structure and function throughout its 

 length, and capable of carrying out all excretory opera- 

 tions. Again, it is a matter of inference that the first part 

 of the proximal tubule in the glomerular nephron may 

 be specifically related to some reabsorptive function, and 

 that with the disappearance of the glomeruli this portion 

 proved to be superfluous and was discarded. 



If one examines the 'evolutionary tree' of the recent 

 fishes it is found that in the primitive or centrally placed 

 families that exclusively inhabit fresh water, or have spe- 

 cies iQ both fresh and salt water, or can migrate freely 

 from one habitat to the other, the glomeruli are rela- 

 tively large and well developed (lungfish, bowfin, carp, 

 goldfish, sucker, catfish, silver perch, bass, sunfish, trout, 

 pickerel, and eel) . In relatively unspecialized but perma- 

 nently marine forms the glomeruli are reduced in size, 

 but they stiU have the appearance of fairly good fimction 

 (sculpin, sea bass, gnmt, squirrelfish, haddock, sea 

 raven, boxfish, porgy, spadefish, triggerfish, and rose- 

 fish). In a third category the glomeruli are small and 

 poorly vascularized, and functional activity is probably 

 greatly reduced (sergeant major, cod, needlefish, biUfish, 

 flying fishes, anchovy, brown tang, puffer, shellfish, and 

 blue-striped grunt) . In the fourth category the glomeruli 

 have disappeared entirely, leaving a purely tubular kid- 

 ney. The extreme aglomerular condition is not charac- 

 teristic of any one phyletic group but occurs in several 

 wholly unrelated groups, though generally in forms 

 which the ichthyologist considers to be highly special- 

 ized—in other words among families distantly removed 

 from the primitive fresh-water stem— such as the goose- 



