Earliest Blood Vessels in Anterior Limb Buds. 303 



arterial sources in the limb's circulation. On the left side, both of 

 the subclavians existing are true segmental vessels, at the eigh- 

 teenth and nineteenth segmental points. 



The cross section (Fig. 11) shows the pair of segmental subcla- 

 vians corresponding to the eighteenth segment, and the correspond- 

 ing dorsal segmentals. The subclavians arise at the dorso-lateral 

 angle of the aortic circumference and, indeed, in a slight local 

 bulging of the aortic wall, from which the dorsal segmental vessels 

 also take origin. The limbs do not project laterally as before but 

 are bent in more, parallel with the main body axis. The aorta is 

 elongated dorso-ventrally with a slight compensatory lateral nar- 

 rowing more marked ventrally so that in section the whole vessel 

 now appears triangular. The dorsal segmental vessels are still con- 

 fined in distribution to the spinal cord and chiefly to its lateral 

 aspect. l!Teither the dorsal nor the ventral surface of the cord are 

 yet supplied with capillaries though these vessels have begun to 

 extend over both of these surfaces. The highest tributaries of the 

 segmental veins are thus now somewhat above the dorso-lateral angle 

 of the cord. 



The remaining embryo of the second period — the period of seg- 

 mental subclavians — is Embryo 11, with thirty-six somites and 

 two segmental subclavians on each side, those of the eighteenth and 

 the nineteenth segments ; but the latter vessels are now mere ves- 

 tigial rudiments. The common origin of the dorsal segmental ves- 

 sels and the subclavians is somewhat more pronounced. 



I need not dwell longer on the four embryos which belong to 

 the period of multiple segmental subclavians. The accounts of 



Fig. 10. — Dorsal view of anterior limb buds autl their vessels in a chick of 

 seventy-two hours incubation, x 53i^ (embryo 9 of table). 



Trans. S. A., transitory subclavian artery, here opposite the sixteenth dorsal 

 intersegmental vessels. The figure shows an interesting stage in the evolution 

 of the limb's vessels. The original subclavian capillaries are now chiefly 

 represented by those at intersegmental points, i. e., the so-called "segmental 

 subclavians." But even here there are atrophying and the chief primary 

 subclavian arteries remain. The latter vessels happen to be constructed from 

 several contiguous subclavian capillaries rather than from a single one as is 

 usually the case. 



