ON THE EARLIEST BLOOD-VESSELS IN THE ANTE- 



EIOE LIMB BUDS OF BIRDS AND THEIR RELATION 



TO THE PRIMARY SUBCLAVIAN ARTERY.* 



BY 



HERBERT M. EVANS. 



Frcnn the Anatomical Lahoratwy of the Johns Hopkins University. 



With 20 Figures. 

 CONTENTS. 



PAGB 



I. Introductory 281 



II. Historical 285 



III. Observations on the origin and character of the first blood-vessels 



in tlie anterior limb buds of chick embryos 288 



IV. Observations on the conditions present at similar stages in em- 



bryos of the duck 308 



V. Comparison with the posterior limb bud 312 



VI. Observations on the early mammalian arm bud 312 



VII. Summary of results 315 



VIII. Application of these facts to the general embryology of the vascular 



system 317 



I. Inteoductory. 



Our knowledge of the origin and method of formation of the vas- 

 cular system in vertebrate embryos is still far from satisfactory, 

 and, indeed, in many instances surprisingly inadequate. This is 

 true, too, in spite of a long series of observations and a number of 

 important contributions in this field. We owe much, for instance, 

 to Rathke, whose classical paper on the aortic arches and the vessels 

 derived from them, dates from 1857, and to Hochstetter, whose per- 

 sistent labors have yielded such a wealth of facts on the develop- 

 ment of various vessels. 



But the knowledge which these and other researches have given 

 us consists chiefly in an idea of the position and connections of the 



*A portion of the observations recorded in this paper were made while occupying 

 one of the research rooms endowed by The Wistar Institute of Anatomy at the 

 Marine Biological Laboratory, Woods Hole, Mass. I would acknowledge here my 

 obligations and thanks to The Wistar Institute for this privilege, and also to Pro- 

 fessor Frank R. Lillie, Director of the Marine Biological Laboratory. 

 The American .Journal of Anatomy. — Vol. IX, No. 2. 



