288 Herbert M. Evans. 



It was from this standpoint, among others, that Professor Mall 

 suggested that we study carefully the earliest arm vessels and it 

 occurred to me that in the embryos of the bird we had a splendid 

 opportunity for delicate injections. The greatly expanded extra- 

 embryonic vessels furnished a good channel of entrance into the 

 circulation of the embryo proper. 



A segmental blood supply to the early limbs must now be regarded 

 as an established fact. Keibel and Elze^^ and the writer^ ^ have 

 shown it to be a normal stage in man, and observations which I have 

 recently been able to make on mammalian embryos show its occur- 

 rence there also. 



However, even the segmental subclavians are not the earliest plan 

 of blood supply to the limb buds, as my injections plainly show; for 

 in the birds, at any rate, the capillaries which first nourish the young 

 limb bud, course at first entirely regardless of any segmental align- 

 ment and in a profusion hitherto unsuspected. 



III. Observations on the Origin and Chakactee of the First 



Blood-vessels in the Anterior Limb Buds of 



Chick Embryos. 



Both chick and duck embryos were employed in the present study 

 and it is for this reason highly probable that the results are gener- 

 ally applicable to the class Aves. Most of my description, however, 

 will be confined to the condition in chicks, inasmuch as it was 

 easier to secure here a larger series of the embryos for study. 

 l^Tevertheless, in view of Eabl's study of the duck, it was important 

 to see if the conclusions reached in the chick could apply here also, 

 and I consequently incubated a considerable number of duck eggs 

 and, at length, obtained successful injections of these embryos. 



The drawings illustrating my findings have been done with the 

 greatest care and fidelity possible. 



I shall report first the conditions obsen-ed in the chick. 



"Keibel u. Elze. "Normentafeln zur Entwick. des Meuschen." Jena, 1908. 



"Evans, Herbert M. "On an Instance of two Subclavian Arteries to the 

 Early Arm Bud of Man and its Fundamental Significance." Anat. Record, 

 II, 9, Dec., 1908. 



