300 Herbert M. Evans. 



but small venules. Thus the earliest drainage channel for the limb 

 does not lose this function during the next succeeding stage, but 

 instead becomes increasingly important as the chief vein of the 

 limb. 



I have presented two typical sections through this embryo in 

 the arm region since these will answer well for the relations thus 

 shown in all the embryos belonging to the first subclavian period. 

 Fig. 8 shows a cross-section through the embryo at the region 

 of origin of the fourth right and the second left subclavians. Both 

 vessels are unsegmentally arranged and hence the dorsal segmental 

 vessels do not appear in the section. While on the right side the 

 subclavian arises from the dorso-lateral angle of the aorta, the left 

 vessel emerges from the true lateral side of the aortic wall, and only 

 a short distance above the origin of the nephric capillaries, one of 

 which is sho-um in the section. Thus in the early stages the place 

 of origin of the subclavians from the aortic circumference varies 

 considerably, and Fig. 4, showing the first left subclavian in this 

 embryo, indicates how far laterally the early vessels of the subclavian 

 series may arise. In this instance the subclavian is almost a mid 

 lateral derivative of the aorta. Such vessels must curve dorsally in 

 crossing the posterior-cardinal vein to reach the tissue of the limb, 

 but the early dorso-lateral branches all course in a straight transverse 

 line.^* The character of the dorsal-segmental vessels and their 

 capillaries has already been mentioned. 



A review of the table which has been presented, shows two features 

 of interest in connection with these earliest stages in the vascular- 

 ization of the limbs. I refer to the high position of the first sub- 



"Rabl has emphasized this straight course of the early subclavians, pointing 

 out that Sabin missed it, for the stages which the latter studied were all old 

 enough to show the dorsal bend which the subclavians then take in reaching 

 the limbs. This dorsal bending is assuredly a secondary bending of an 

 original straight vessel ; but my own specimens have disclosed a number of 

 the very earliest subclavians arising from so low a point on the lateral 

 aortic wall that a primary arching course is necessary to reach the limb 

 tissue. It is not unlikely that these subclavians disclose the more primitive 

 place of origin of the subclavian series, for they do not occur in even 

 slightly older embryos. Good justification thus exists in considering the 

 subclavians as primarily true lateral branches of the aorta. 



