304 Herbert M. Evans. 



Rabl and of Miiller have already sufficiently emphasized this inter- 

 esting stage in the limb vessels. 



We have seen that most of the subclavian capillaries arising from 

 the aorta at non-segmental points eventually atrophy, and there 

 now remain only the vessels which stand opposite the segmental 

 interspaces. Thus are produced the segmental subclavians, a truly 

 metameric arrangement of the limb vessels. 



Two features of some importance in these stages have been pre- 

 viously overlooked. These are: 



1. Abundant traces of the earlier capillary plexus stage of sub- 

 clavians occur in the period of segmental subclavians. These con- 

 sist in several smaller or atrophying vessels of the subclavian series 

 which do not stand at segmental points. Such vessels are often 

 present to complicate the picture of the segmental subclavians, espe- 

 cially early in this period. When the stage is reached in which the 

 multiple segmental subclavians are carried up as common branches 

 with the dorsal segmental vessels, these non-segmental rudiments 

 rarely persist longer and we have at length a perfect picture of mul- 

 tiple true segmental subclavians. 



Most of the non-segmental subclavians of this stage are delicate 

 vessels, but it occasionally happens that some of them are larger 

 sturdy channels of equal value with the segmentals. This was the 

 case, for instance, in Embryo 10 of the series. It was thus sur- 

 prising to me that Rabl had not found such vessels, but a careful 

 rereading of his descriptions shows that he doubtless saw some in- 

 stances of them. He attached a peculiar significance to them, how- 

 ever, conceiving that they came about through a splitting of a pre- 

 existing single segmental vessel, thus forming a double vessel whose 

 roots wandered apart ! It was quite impossible to him that the 

 subclavians should arise at other than segmental points. Even the 

 cases of "insel bildung" he would make come through a similar 

 splitting of single vascular channels. 



2. The second point which I wish to make is that it must occa- 

 sionally happen that even the vessel most favored in the row of sub- 

 clavians may not be at first at an exactly segmental point as Fig. 

 10 plainly showed. 



