VASCULAR AREA OF CHICK BLASTODERM. 



8 9 



fore, must be located in the primitive streak mesoderm. To 

 attribute such a power to it is not inconsistent with other facts, for 

 it develops blood-islands in the area pellucida, where there is no 

 germ-wall ; but for the rapid and abundant development of islands 

 the mesoderm must come in contact with the germ-wall, where it 

 receives an abundance of food-stuffs. If, however, the germ- 



m 



*m^ 



FIG. 6. The resulting embryo in Experiment III. (see Fig. i, C). The 

 operation was made after nine hours of incubation, and the egg was then 

 incubated for thirty-four hours. The region destroyed by the operation is 

 represented by a large hole, opposite to which the vascular area is wanting. 

 Posterior to the hole is a small area of vascular tissue, which doubtless has been 

 formed from the part of the primitive streak material overlapping the area 

 opaca at the posterior border of the area pellucida. The anterior part of the 

 vascular area is slightly asymmetrical, owing to the retardation of the spread- 

 ing of the mesoblast on the left side, caused by the injury made on this side 

 in the area opaca. X 5.4- 



FIG. /. The resulting embryo in Experiment IV. (see Fig. i, >). The 

 operation was made after about seven hours of incubation, and the egg was 

 then incubated for fifty hours. The embryo is entirely wanting, but the 

 vascular area is well developed, except at the posterior end where its spreading 

 has been retarded by the most posterior injury. The lateral parts of the 

 vascular area consist of a net-work of vessels, and in the region of the pellucid 

 area large vessels pass in towards the center. No heart, however, is de- 

 veloped. X 5.8. 



wall elements actually develop into vascular tissue, they must 



receive their initiating stimulus from contact with the mesoderm. 



In addition to supporting the above contention, the results ob- 



