246 ORIGIN OF BLOOD-VESSELS IN BLASTODERM OF CHICK. 



this area by McWhorter and Whipple in their study on the living chick blastoderm. 

 These authors studied them from the standpoint of the origin of the vessels from 

 tissue-spaces. They are, on the other hand, formed from angioblasts by the lique- 

 faction of the center of the mass and will join the main mass of the vessels by the 

 process of sprouting. At the stage of 5 somites these masses of angioblasts are all 

 solid. Over the middle zone of the area pellucida, which will become the area of 

 the vitelline veins, there is an extensive mass of solid angioblasts, becoming less 

 extensive farther posteriorly. This illustrates that the middle zone opposite the 

 venous area of the heart is always the precocious area in the development of the 

 vascular system in the chick. The myocardium is just indicated in this specimen, 

 and a very few angioblasts, which are the first forerunners of the heart, may be 

 seen between the myocardium and the endoderm, both in the total specimen in 

 figure 6, and in sections from the same stage. Their position is indicated in the 

 diagram of figure 1 in the text, from a chick of 6 somites. No angioblasts along the 

 margin of the somites in the position of the future aorta can be made out in this 

 particular specimen, but they could be in the other specimens of this stage. 



Thus, to sum up the stage of 5 somites, it is important as showing the first 

 blood-vessels from the primitive angioblasts, making an outer rim of vessels over 

 the entire area opaca, the forerunner of the marginal sinus. It marks thus the 

 stage of the first blood-plasma and of the first erythroblasts which can be identified 

 in the form of blood-islands in these primitive vessels. It is the stage in which 

 angioblasts are first found in any great numbers in the area pellucida, as well as in 

 the stage in which the first angioblasts can be seen in the axial line, constituting the 

 forerunners of the endocardium and the aorta. 



During the stages of 6, 7, and 8 somites very interesting changes take place. 

 These are illustrated in a blastoderm with 7 somites in figure 7. In the area opaca 

 the blood-vessels of the outer rim have developed great masses of blood-cells. 

 These are no longer in the form of blood-islands attached to the wall, but have 

 become free cells which in many places, and especially at the outer margin, com- 

 pletely fill the lumina of the vessels. In the inner margin of the area opaca, on the 

 other hand, the angioblasts are just beginning to pass into the stage of liquefaction 

 to form vessels. 



The area pellucida likewise shows very interesting features. Over the amino- 

 cardiac vesicles, especially of the left side, can be seen fine parallel lines of angio- 

 blasts, which are also characteristic of this area. These have been retouched in the 

 photograph. They are on the ventral surface of the amino-cardiac vesicles and are 

 the forerunners of the anterior veins of Popoff. They are especially interesting 

 because one can always find in them examples of the liquefaction of the cytoplasm 

 in single chains of angioblasts, as shown in figure 25, plate 5, which seems to me to 

 be the best proof that the lumen of the vessels may be considered as intracellular. 

 This specimen contains a number of isolated clumps of angioblasts over the dorsal 

 surface of the amino-cardiac vesicles and even extending into the middle zone of 

 the area pellucida. These clumps of angioblasts dorsal to the somatopleure, which 

 are particularly abundant in this specimen, are constant. They are shown in Lillie's 



