Vol. XVI. February, 1909. No. j. 



BIOLOGICAL BULLETIN 



AN EXPERIMENTAL STUDY ON THE DEVELOP- 

 MENT OF THE VASCULAR AREA OF THE 

 CHICK BLASTODERM. 1 



J. THOS. PATTERSON. 



The question regarding the source of the cell-layer in which 

 occurs the formation of vascular tissue of the opaque area of 

 the bird blastoderm has attracted much attention on the part of 

 students of avian embryology. While the solution of this prob- 

 lem has been sought almost entirely through the study of sections, 

 yet this method of attack is met with many difficulties. It is 

 almost impossible to determine by histological means the exact 

 part played by each of the germ layers in the formation of the 

 vascular tissue of the area opaca. It is not my purpose to 

 discuss the extensive literature that has grown up around this 

 problem ; but I shall simply state the two views that are com- 

 monly held with reference to the origin of the mesoblast, out of 

 which arises the vascular tissue of the area opaca. According 

 to one view it is derived from the germ-wall, by a delamination ; 

 according to the other, it is merely a peripheral extension of the 

 primitive streak mesoblast. If the former view be correct, it is 

 obvious that the germ-wall ought to give rise to vascular meso- 

 blast, irrespective of the presence of the primitive streak meso- 

 blast. In other words, if the primitive streak mesoblast were 

 prevented from growing out over, and coming in contact with 

 the germ-wall, the latter ought still to give rise to vascular meso- 

 blast. 



Experiment I. The experimental test for this proposition is 

 a very simple one, and consists in making injuries on an unin- 



1 Contributions from the Zoological Laboratory of the University of Texas, 

 No. 95. 



83 



