DEVELOPMENT OF PRIMITIVE BLOOD-VESSKLS. 65 



an organ to the heart, provided it does not serve as the capillary bed for that organ or 

 break up into another capillary plexus. Such vessels I have colored blue. All 

 other vessels I have indicated in gray, and shall try to trace the complicated changes 

 which such vessels undergo, serving at times as arteries, veins, or capillaries, or as 

 vessels with a double function. This method of nomenclature is therefore based on 

 the function of the vessel at the stage when it exists. It takes into account the very 

 shifting course of the blood, as the vessels develop in the embryo better than does 

 the method of making a too early identification of the adult vessels in those of the 

 embryo. This usage of terms serves to restrict especially the term vein as applied 

 to the embryo. The meaning of the terms artery and vein, as applied to the embryo, 

 will become more clear in the discussion of individual vessels. 



METHODS. 



As has been stated, in this study I have followed two methods: first, that 

 of the injection of embryos, and second, the method of studying the living blasto- 

 derm in the case of the chick in a hanging-drop preparation. A general account 

 of the methods of injecting embryos will be found in my paper on the ayzgos veins 

 published as "Contribution to Embryology, No. 7," by the Carnegie Institution 

 of Washington in 1913. All of the injections of young embryos are made by 

 blowing ink into the vessels through a very fine canula. 



To inject the young chick the shell is opened and the embryo exposed to a 

 strong light under a binocular microscope. A few drops of warm Locke's solution 

 are placed on the blastoderm, and the vitelline membrane is removed. By the 

 time the chick has 14 somites the sinus terminalis, or marginalis, is well developed 

 in the edge of the area vasculosa and can easily be punctured with a fine canula. 

 Nevertheless it is not easy to obtain complete injections of the blood-vessels of 

 the chick through the veins until the embryo has about 16 somites. In stages 

 between 9 and 16 somites more complete injections can be made by puncturing 

 the aorta directly. This is a very interesting point in connection with the time 

 of the beginning of the circulation. I shall show that, though the heart com- 

 mences to beat about the time the tenth somite is forming, the circulation does 

 not begin until about the stage of 16 somites. From the time the circulation 

 begins it is easy to get complete injections by blowing a little ink into the vitelline 

 veins and allowing the heart to pump the ink through the vessels of the embryo. 

 If total specimens are desired it is well to dilute the ink one-half, so that the 

 superficial vessels will not become so dense as to obscure the deep ones. I shall 

 discuss in this paper the effects of injection in the embryo before the circulation 

 has begun. 



The earliest chick embryo which I have injected was one of 9 somites; and 

 I believe this stage to be about the youngest to which the method is applicable. 

 At the stage of 6 somites the dorsal aorta is in the stage of a plexus of angioblasts, 

 many of which are still solid cells. This plexus of cells gradually acquires a lumen 

 and becomes the aorta during the stages of from 6 to 9 somites. All stages up to 

 18 or 19 somites can be studied to great advantage in hanging-drop preparations. 



