344 OSCAR RIDDLE. 



capacity to form methaemaglobin in the blood and thus reduce 

 the oxygenation of the tissues is a fact which should not be 

 overlooked. But, if it be granted that a reduction by this means 

 of the supply of oxygen to the tissues has a tendency to pro- 

 duce defects, then it becomes plain that a reduction of the blood 

 pressure alone must tend to produce them also, since in any lower- 

 ing of the blood pressure the amount of oxygen available to the 

 tissues is decreased. It would seem moreover that we are justi- 

 fied in throwing out of consideration all other actions of amyl 

 nitrite than those connected with the blood, since the inferior 

 umbilicus of a feather-germ is a portal through which there 

 enters from the body practically nothing except the blood- 

 stream. 



The Homologies of the Feather Defects or Fault-Bars. 



The conclusive proof which has just been given that the fault- 

 bars of feathers are produced by a reduced nutrition puts us in a 

 position to state positively that these defects are the homologues 

 of other defective growths of epidermal structures long known 

 to be thus caused. Among these may be named : imperfections 

 of the enamel of the teeth (dental hypoplasia) ; the grooving of 

 the nails after illness ; certain changes in the hair, particularly 

 the weakening of the fibers of the wool of sheep which had been 

 underfed ; and the rings on the horns of many ungulates. It 

 has already been suggested by Duerden that these structures are 

 similar in nature to the feather defects. The proof of the sug- 

 gestion lies, however, as stated above, in the determination of the 

 cause of the fault-bars. 



There should be added to this list such similar markings of 

 epidermal structures as the concentric rings of annual growth 

 found on the shields of many tortoises, and probably also the 

 stratified hoof-formations of many animals. 



It is worthy of note that the range of time involved between 

 two of these successive abnormal productions varies from one 

 day to one year. In the feather, however, the time varies only 

 from one day to several days — different types of fault-bars re- 

 sulting from the different periods. There is, nevertheless, but 

 one cause for all these various formations, namely, a reduced 

 food-supply, and this warrants our grouping them together. 



