Earliest Blood Vessels in Anterior Limb Buds. 311 



capillaries should chance to be opposite the intersomitic spaces as 

 are opposite the somite masses and vice versa. 



Duck embryo 2, possessing thirty-eight somites, happens to have 

 almost as many subclavians as occurred in the younger embryo, but 

 in the older stage, besides being larger, these vessels are almost all 

 at segmental points, so that the embryo belongs clearly to the period 

 of multiple segmental subclavians. 



On the right side, there are six subclavian vessels arising from 

 the aorta. The first subclavian occurs just in front of the six- 

 teenth segmental vessel, the second and third at the level of the 

 seventeenth segmental vessel, the fourth and fifth at the level of the 

 eighteenth segmentals, and the sixth halfway between the eighteenth 

 and nineteenth segmentals. 



On the left side, the first subclavian stands opposite the sixteenth 

 segmentals, the second opposite the seventeenth vessels, the third 

 opposite the eighteenth segmentals, and the fifth opposite the nine- 

 teenth segmentals. Thus there are on the left side as many as four 

 segments represented by subclavians. 



However the study of even this embryo, with such a complete 

 series of segmental subclavians, shows that here also there persist 

 some vessels not in segmental alignment. The last subclavian on 

 the right side is such a vessel, for it occurs midway between the 

 eighteenth and nineteenth dorsal segmental vessels. The cases of 

 two subclavians existing opposite a segmental point are easily ex- 

 plained by the chance origin of two of the early capillaries opposite 

 one of the inter-somitic clefts. In such cases both vessels are 

 equally favored, and both persist to the stage of segmental subcla- 

 vians, where they increase the number of vessels to be expected. 

 1 have no doubt but that the condition in this Embryo 2, was pre- 

 ceded by a stage of some ten or twelve subclavian capillaries, similar 

 to those seen in chicks 3 and 4, but these interesting stages are so 

 transitory in character that it is only rarely, that we have the good 

 fortune to see them. Some capillaries, here as elsewhere in the 

 developing vascular system, push out, function slightly and die 

 in a surprisingly short time. 



