350 OSCAR RIDDLE. 



From the above described relations it is plain that in case of a 

 poor blood supply to the germ, the distant (more external) bar- 

 bules would be first to suffer, since all their nutriment must filter 

 through the cylinder-cell layer and around the barbs in order to 

 reach them. The cylinder cells by their shape (usually spindle- 

 shaped) are especially adapted to take up a maximum amount of 

 food, so that parts peripheral to these will certainly suffer first. 



It now becomes evident that the low blood pressure produced 

 by amyl nitrite, and the low pressures (discussed below) occcur- 

 ring normally, must exercise their chief effects on the distant 

 barbule cells ; a flow of lymph from the latter occurring as soon 

 as the capillary tension is reduced. It is also plain that such a 

 movement of the lymph must act upon these rapidly dividing 

 cells in the same way as would an actual reduction of food with 

 the blood pressure remaining constant. It is upon extremely 

 actively dividing cells, which are removed by several cell diam- 

 eters from the capillaries, that changes in the vascular pressure 

 have an opportunity to exert an influence. Could more favor- 

 able conditions be imagined ? 



This daily rhythm of accelerated and depressed mitosis recalls 

 similar cases in plants. It is well known that the cells of the 

 root-tips of the Windsor bean ( Vicia faba) show the greatest 

 mitotic activity at noon — 1 1:00 to I :oo daily. Spirogyra shows 

 most rapid division at 1:00 A. M. and this period of maximum 

 activity may here be delayed for several hours by cooling. In 

 the plants it is difficult to determine the actual cause of the 

 rhythm. In the feather cells, however, it is certain that the period 

 of fewest mitoses is the period at which least food is available. 



Blood Pressure Rhythms in Birds. 

 It must be said that a daily blood pressure curve for birds has 

 not yet been produced. Probably no one, however, will doubt 

 that there is a rhythm of high and low pressures in birds. Prob- 

 ably too, physiologists generally would expect the lowest point 

 of the curve to correspond to the night — as in mammals. In 

 fact, what we know of the factors upon which blood pressure 

 depends in mammals, and what we know of the structures, habits 

 and physiology of birds, would compel us to ascribe to the latter 



