DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 69 



in the somatopleure, and a more abundant sheet in the splanchnopleure. Because 

 of the development of the head-fold and the heart, it was impossible that the 

 approach of the vascular plexus should be uniform over the entire length of the 

 embryo. For example, the most cephalic part of the head became cut off from 

 direct lateral connection with the embryonic membranes, and the vessels which 

 approach the heart gradually rotated from the direct transverse direction to an 

 oblique angle. He then noted that the dorsal aorta developed in the mesial edge 

 of the plexus of angioblasts of the area vasculosa along the line of the lateral edge 

 of the myotomes. From these observations he concluded that the vessels of the 

 embryo are derived from the vessels of the membranes, and that the portion of the 

 axis which can not be seen to receive the plexus of primitive angioblasts from the 

 membranes receives its vessels by a growth of the plexus which has already invaded 

 the embryo at other places. 



In following the differentiation of the vascular area by improved methods 

 whereby one can watch the living cells growing under an oil-immersion lens, it is 

 astonishing how accurate is this description of His, which must have been made by 

 far cruder methods. To his description must be added that with finer methods it is 

 seen not only that the plexus out of which the aorta develops is the border of the 

 common plexus of the entire area vasculosa, but that new cells differentiate along the 

 axis of the embryo as well, so that angioblasts differentiate over the entire zone from 

 the outer edge of the area opaqua to the margin of the future aorta along the lateral 

 border of myotomes. Thus His's description must be extended to include a dif- 

 ferentiation of new angioblasts in the axis of the embryo itself. 



In the living blastoderm over the area opaqua, the endoderm-cells are so 

 thick and so filled with yolk that the development of the blood-vessels and the 

 blood-cells beneath them can be followed only with great difficulty. In the area 

 pellucida, on the other hand, the endoderm is thin, and during the periods when 

 the endoderm cells are not dividing they are so clear that it is easy to focus 

 through them. 



At the stage of 6 somites the head-fold is well formed and the amnio-cardiac 

 vesicles have met in the mid-ventral line. Along the axis of the embryo there is 

 a zone of dense tissue radiating from the primitive streak and from the embryo 

 cephalic to the streak. This denser mass of tissue divides the area pellucida into 

 an inner thicker zone containing the axis of the embryo and an outer thinner 

 zone. In sections it is clearly seen that this denser zone is due to the further 

 development of the ccelom nearer the embryo. Over the cephalic part of the 

 denser zone the coelom has a wide lumen, and both its ventral mesoderm and 

 the endoderm are thicker than the same membranes farther lateralward. This 

 is very plainly shown in Duval's Atlas, plate xiv, figure 218, in the zone extend- 

 ing outward from his letter b. Farther caudalward in this dense band, opposite 

 the undifferentiated myotomes and the primitive streak, there is no cavity of the 

 coelom, and its dorsal and ventral mesoderm are fused and form a dense mass of 

 cells. This entire thicker zone is difficult to study in the living chick, but the 

 whole outer margin of the area pellucida is clear and the cells are so thin that 



