VASCULAR AREA OF CHICK BLASTODERM. 



been excluded as a result of the operation. The portion of the 

 germ-wall in the region of the section does not differ histo- 

 logically from that lying beyond the anterior or posterior limit 

 of the vascular area, yet it is at least twenty-four hours beyond 

 the time when it should have given rise to vascular mesoblast 

 (if the first view stated above be correct). Nor does it give any 

 evidence of so differentiating later, even in experiments in which 

 development has been allowed to go on as long as seventy-two 

 hours. This makes it practically certain that in the absence of 

 the primitive streak mesoblast, the germ-wall does not give rise 

 to vascular mesoblast. 



Experiment II. A less conclusive experiment than the pre- 

 ceding consists in making injuries out in the area opaca (Fig. 



FIG. 2. Photograph of the resulting embryo in Experiment I. The opera- 

 tion was performed on the unincubated blastoderm (see Fig. i, A), and the 

 egg was then incubated for forty-eight hours. The embryo is well developed 

 with twenty-four somites well differentiated. The lateral expansion of the 

 vascular area has been prevented by the two injuries. X 5.8. 



FIG. 3. Outline sketch of the embryo shown in Fig. 2. The broken lines 

 at the sides indicate approximately the limits to which the lateral parts of 

 the vena terminals would have reached had the blastoderm been uninjured. 

 X 4-4- 



