114 JOHN LEWIS BREMER 



acteristics of the blood-islands, in that they also change from 

 solid to hollow independently; but in the angioblast cords there 

 is no sign of the formation of blood-cells. It was the observance 

 of these isolated spaces, which later fuse to form large vessels, 

 that lead to the often repeated statements of Ruckert and Mollier 

 and others that the dorsal aortae arise in situ from the cells of 

 the mesoderm; and in truth the connection with the lateral 

 capillary net is short lasting and sometimes extremely tenuous. 

 Tiirstig ('84, 1) recognized the presence of solid cords leading a 

 short distance from these hollow spaces, but did not trace their 

 connections; others of this school have missed them entirely. 



The story of the development of the primary arterial system 

 can best be told with the aid of the figures. Fig. 1, a reconstruc- 

 tion of the angioblast cords of one side of a rabbit embryo of 

 five segments, shows these cords, streaming in from the network 

 over the yolk-sac, the cut ends where the reconstruction was dis- 

 continued showing at the right border of the figure. They have 

 grown from right to left of the figure, occasionally anastomosing, 

 until near the median line, which lies at the left of the figure. 

 The shape of the meshes of this net indicates, it seems to me, the 

 direction of this growth, and the rapidity with which it has oc- 

 curred. Cephalad the net is limited by the proamnion, where no 

 mesoderm exists; caudad the net continues beyond the portion 

 drawn. In a few places the cords have become hollow; here and 

 there near the median line, and especially at the right of the figure, 

 where a considerable chain of hollow spaces extends longitudin- 

 ally, near the lateral border of the embryo proper. This chain, 

 the future lateral heart, lies beneath the coelom, and like the 

 coelom at this stage is situated only in the anterior third of the 

 embryonic body. At the left of the figure the net ends rather 



Fig. 1 Rabbit, 8§ days, 5 segments, 3.4 mm., H.E.C. no. 650. Reconstruction 

 of the angioblast of the left side, anterior two-thirds of the embryo, seen from the 

 entodermal cavity. The median line of the body is to the left of the drawing, solid 

 cords of angioblast come from the yolk sac on the right. Hollow vessels are indi- 

 cated by the shading. Brackets show position of somites; arrows mark regions 

 where the aortic network is as yet incomplete'; x - y indicates plane of section of 

 fig. 2. X 150. 



