Earliest Blood Vessels in Anterior Limb Buds. 291 



that between the tirst and second myotomes) is not occupied by a 

 dorsal segmental artery. The series of dorsal segmental arteries in 

 these embryos begins ivith the vessel present in the interspace between 

 the second and third somites. Consequently the vessels present in 

 the eighteenth interspace are really the seventeenth pair of the series 

 actually present, and I have so labelled them in all the drawings. It 

 is to be borne in mind, then, that the seventeenth segmental vessels 

 of the figures are in the eighteenth inter-somitic septum. Inasmuch 

 as the first four somites of the chick are to be considered cephalic, 

 rather than cervical, in their ultimate fate, and as the third actual 

 segmental artery later courses near the first cervical nerve, and is 

 hence the first cervical artery, the primary subclavian artery arises 

 from the fifteenth cervical segmental artery, a vessel occurring in 

 the eighteenth interspace and the seventeenth of the series actually 

 present. 



There are five periods in the history of the bird's subclavian and 

 not four as Eabl maintained. These may be briefly enumerated as 

 f ollow^s : 



I. Period of capillary outgrowth from the aorta forming a pri- 

 mary-limb plexus, not influenced in its arrangement by metamerism. 



II. Period of multiple segmental subclavians, a condition result- 

 ing from the atrophy of all capillaries in the preexisting plexus not 

 at segmental points. 



III. Period of the establishment of the primary subclavian artery 

 from the persistence of one of the pairs of segmental subclavians — 

 i. e., that of the eighteenth segment. 



IV. Period of double arterial supply through contemporary ex- 

 istence of dorsal and newly arisen ventral subclavians. 



V. Period of enlargement of the permanent channel, the sec- 

 ondary subclavian, and coincident atrophy and disappearance of 

 the primary vessel. 



The last three periods or phases were described by Hochstetter 

 and Sabin, the second period, in which segmental subclavians exist, 

 by Eabl and Mliller, the earliest or first period for the first time 

 in the present study. 



