ORIGIN OF VASCULAR TISSUES 99 



The heart of this embryo (fig. 63) is seen to be solid in its an- 

 terior portion. More posteriorly (fig. 62) in the arterial end the 

 endothelium has a lumen. The endothelium again becomes 

 solid immediatelj'' at the 'insertion' of the arterial end of the 

 heart on the embryonic body. A section passing through the 

 posterior limits of the diencephalon shows well developed pre- 

 cardinal veins (fig. 64) so closely packed with eiythrocytes that 

 the latter have angular contours. Still more posteriorly in the 

 region of the otocysts (fig. 75) the precardinal lines are seen to 

 contain erythrocytes. This section also passes through the very 

 small portion of the ventral aorta which was able to form. Al- 

 most immediately posterior to this plane of section, the aorta 

 completely disappears. Besides this small portion of ventral 

 aorta, no portion of the branchial arterial system was found in 

 section. The remainder of the ventral aorta, the aortic arches 

 and cephalic dorsal aortae are entirely absent. Since no vascu- 

 lar tissues developed in the anterior yolk, and no erythrocytes 

 were formed in the heart of this embryo it seems reasonable to 

 assume that these erythrocytes were local formations in the ven- 

 tral aorta. They may or may not have been contributed to by 

 aortic endothelium. 



This embryo received closer observation than most of the em- 

 bryos studied, for the reason that its peculiar eye-formation ren- 

 dered it conspicuous even before the blood could possibly have 

 circulated. While a number of mesenchyme cells reached the 

 anterior yolk, no formation of endothelium or erythrocytes took 

 place in this region. This fact was noticed in the living condi- 

 tion and is verified in section. Some of the mesenchyme cells 

 on the anterior yolk would probably have transformed into 

 erythrocytes if the embryo had been allowed to live longer; 

 some of them, still typically stellate, were found to be very 

 slightly eosinophilous. In figure 62 the yolk sac is devoid of 

 vascular tissues. The same is true of all sections anterior to 

 this, and of many which are posterior. Figure 75 shows the 

 most anterior extent of erythrocytes that were found on this 

 yolk-sac. These were far dorso-lateral, and belong to an iso- 

 lated lacuna. 



