Franklin P. Mall. 233 
early date in which no arterial and venous channels can be differentiated 
(Fig. 2). A few channels are, however, selected by the blood-stream in 
consequence of the general direction which is given to it by the position 
of the ends of the primitive aorta on the one side, and of the venous ostia 
of the heart on the other. These channels (Fig. 2, a, 6, c) contain the 
more rapidly flowing streams. They, therefore, dilate and become con- 
verted into arteries and veins. (Fig. 3). 
* Other channels, in which the rate of the flow of the blood has a cer- 
Fie. 2. Capillary channels of the area visculosa after forty-eight hours’ 
incubation. S, peripheral end of the primitive aorta; a, b, c, selected blood- 
channel, X 30. After Thoma. 
tain medium force, remain as capillaries, and lastly, some channels which 
offer great resistance to the stream, and are thus very slowly traversed, 
atrophy, or disappear altogether. The rapid growth of the selected chan- 
nels diminishes the resistance to the blood-stream, so long as the capillary 
area remains unaltered. The blood-pressure in the capillary area accord- 
ingly rises and leads to new formation of capillaries. New communica- 
tions are thus formed between the terminal ramifications of the arteries 
and veins; the capillary_area is thus relieved, and its blood-pressure falls. 
Arteries and veins have now become wider and longer, and the capillary 
