Earliest Blood Vessels in Anterior Limb Buds. 301 



clavian capillaries — their origin in the neighborhood of the twelfth 

 and thirteenth segments — and to certain differences in the vascular- 

 ization of the right and left limbs. 



RabFs studies indicated that the subclavians of later stages 

 arose at successively lower levels from the aortic wall. The injec- 

 tions here reported, however, indicate a more cephalic extension 

 of the subclavians than had been previously suspected, and so extend 

 even more the "wandering" of the upper limb and its vessels. 



It appears that the right limb bud is the first to receive capil- 

 laries, and that this limb in the early stages always possesses a greater 

 number of subclavian capillaries than the left limb. Apparently 

 the two limbs are identical in their relations to the body and in the 

 conditions with which they have to deal in their development save 

 in one respect, namely, that at the time of origin and earliest 

 stages of the limb buds, the embryo is always resting on the left 

 side. This causes a slightly more flexed position of the under or 

 left limb-bud with reference to the body wall and permits a some- 

 what extended, freer projection of the uppermost right limb. This 

 may be related to the greater speed and profusion with which the 

 first vessels grow into the right limb. 



But in summing up the condition found in the five embryos illus- 

 trating the first period in the development of the subclavian, noth- 

 ing is more striking than that we have to deal here merely with an ir- 

 regular plexus of true capillary vessels luhich are in no way related 

 to a segmental plan. Thus if the chance arrangement of any irregu- 

 lar capillary plexus obtains here, it should happen that as many of 

 the vessels arise from non-segmental as from segmental points, and 

 this is actually the case. 



Embryos of the Second Period. 

 The first embryo classed in the stage of segmental subclavians 

 (Embryo 8), has almost as high a proportion of non-segmental ves- 

 sels as has Embryo 7, but two sigiiificant changes have occurred. 

 These consist in the purely rudimentary character of the non-seg- 

 mental subclavians and the enlargement of that pair of segmental 

 subclavians opposite the eighteenth pair of dorsal segmental ves- 



