440 Helen Williston Smith. 



pigs of various stages of development are available every day. In 

 the latter case the injection of India ink is made into the um- 

 bilical artery towards the heart which, while beating, enables one 

 to obtain perfect injections. 



Studies in this laboratory have shown conclusively that the entire 

 vascular system is developed from a common plexus of capillaries 

 which gradually extends over the whole body, a part of which is 

 transformed into arteries and a part into veins. Throughout de- 

 velopment the vessels are functioning, and the formation of arte- 

 ries and veins is only an expression of the law of functional adapta- 

 tion of the extensive capillary plexus peripheral to a beating heart. 

 How could it be otherwise, for the arteries and veins are not formed 

 step by step by sjorouts, or by the union of independent anlages, but 

 they are functioning from the time of their simplest beginning 

 until the animal dies. 



The present study was undertaken, at the request of Dr. Mall, in 

 order to follow, in a relatively simple field, the gradual evolution 

 of the vascular system from its first appearance until its adult form 

 is reached. This study is one of a series, two of which by Dr. 

 Evans are now in press, and others are in preparation. If circum- 

 stance will permit I hope to follow this with an account of the 

 development of the deeper vessels of the body wall.^ 



We find in the earliest stages considered in this jiaper a relatively 

 simple circulation in the body wall, one in which the posterior 

 cardinal and the umbilical veins are both formed, but have com- 

 paratively few ramifications. As the embryo grows the limb buds 

 appear and the membrana reuniens closes in around the umbilical 



'The literature on the standpoint taken in tliis study is as follows : Aeby, 

 Der Bau des menschl. Korpers, 1871; Baader, Inaug. Diss., Bern, 1866; 

 Ttioma, Untersuch. iiber die Histogenese und Histomecbanik des GefJiss- 

 systems, 1893; Flint, Johns Hopkins Hospital Reports, IX, 1900, Amer. Jour. 

 Anat., II, 1903, and VI, 1907 ; Mall, Amer. Jour. Anat., IV., 1905, and V, 1906 ; 

 Rabl, Arch. f. m. Anat., LXIX, 1907; Evans, Anatomical Record, II, 1908, 

 and Amer. Jour. Anat., IX, 1909. 



The specimens on which the present article is based were demonstrated at 

 the last meeting of the Association of American Anatomists, Baltimore, Decem- 

 lier, 1908. 



