Earliest Blood Vessels in Anterior Limb Buds. 307 



I have placed it there since it is the only one which does not fit 

 well into the series, for though its age and number of somites would 

 indicate a more advanced scheme in the vessels of the limb, I 

 found here no less than six vessels of the subclavian series. The 

 embryo may be viewed, as an instance, in which the limb and its 

 vascular system has run slightly behind the normal for this age, or 

 as a case of the persistence of multiple non-segmental subclavians. 

 I have drawn the embryo, as a whole, since it shows splendidly the 

 typical relations of the early limb capillaries and those belonging 

 to the dorsal segmental series (Fig. 3). 



Pekiod of Primary Subclavian Artery. 



Embryos 12 to 16 all illustrate stages in the growth of the pri- 

 mary subclavian artery. The relation of this trunk to the preexist- 

 ing segmental subclavians, has already been clearly indicated in 

 Embryo 8 ; even there we saw the early exaggeration of one of the 

 members of the subclavian series. The processes of vascular atrophy 

 and death which early eliminate the original non-segmental sub- 

 clavians, destroy also eventually the segmental subclavians with the 

 single exception of that vessel destined to become the primary sub- 

 clavian artery. 



It is of interest that even in these late stages, there sometimes 

 persists a non-segmental artery. Fig. 12 gives an instance of this. 

 It is the Embryo 12 in the series and possesses some forty-five 

 somites. The large primary subclavian artery has below it and near 

 the middle of the adjoining somite, a narrow rudimentary vessel 

 which has persisted from the primary subclavian series. True 

 segmental vessels, other than the main one, may likewise persist 

 in limbs of this age. This is possible through the early proximal 

 anastomoses between subclavians. The use of several of these paths 

 will give several segmental roots of origin to the primary subcla- 

 vian trunk. Fig. 12b, is a striking instance of this. 



In embryos of eighty-four and ninety-six hours incubation, the 

 primary subclavian artery has attained a large size. The dorsal 

 segmental vessels have also increased in caliber. The capillaries 

 belonging to the latter system have surrounded the spinal cord com- 



