174 OSCAR RIDDLE. 



more removed. Now this is exactly what happens. The cap- 

 illaries (Fig. 3 cap] of the feather-germ lie nearest those cells 

 which enter into the formation of the barbs (Fig. 3, b] and these 

 are able to continue to grow even with a weakened food-supply ; 

 they too, though, are suppressed in cases of extreme starvation. 

 The cells which form the barbules (Fig. 3, ft) are not in contact 

 with capillary walls, and can utilize only the surplus of food which 

 filters through the barb-forming cells. With this fact in mind it 

 is clear that we should expect a diminished food-supply to first 

 check the growth in the barbules and that still further reduction 

 is necessary to check the growth of the barbs. Experience proves 

 that this is true (I use the words " food " and " nutriment " in a 

 broad sense, and oxygen is to be read into them). It is conceiv- 

 able that a reduced oxygen-supply is here playing a part, since 

 in all my experiments and in any normal lowering of the blood- 

 pressure, the available oxygen is decreased. 



From what has just been said of the filtration method by which 

 the barbule cells receive their nutriment, we can now see how it 

 is that blood-pressure plays so important a part in the produc- 

 tion of defective areas. It is well known that when a period of 

 low blood-pressure sets in, the lymph begins to flow from the 

 spaces between the cells of the body into the capillaries. Thus, 

 by withdrawing a quantity of food from the immediate environs 

 of the cell, a low blood-pressure affects the cell in the same way 

 as does an actual reduction of the amount of nourishment circu- 

 lating in the blood. 



It appears, therefore, that the feather germ as it grows and 

 unfolds, spreads before us a record of some earlier significant oc- 

 currences within. Indeed, it now seems certain that the delicate 

 filaments, so admirably interlaced to form a feather-vane, are as 

 capable as a revolving drum of recording the important changes 

 in vascular pressure. To be sure, the tracings on the plumes are 

 not to be measured as symmetrical curves with a definite num- 

 ber of millimeters of daily variation, but are written in "funda- 

 mental bars " separated by areas more or less imperfect which are 

 to be read in terms of cell-growth and cell-division. 



HULL ZOOLOGICAL LABORATORY, 

 UNIVERSITY OF CHICAGO, 

 December, 1906. 



