ORIGIN OF VASCULAR TISSUES 



63 



b. Complete isolation of meroplasts. In general the methods of 

 operation have fallen under two types: Experiments designated 

 as Type I are those in which a small portion of the embryonic 

 body was completely isolated from the outlying blastoderm. 

 An examination of figures 1 and 2 will explain the methods by 

 which this complete separation was accomplished. Chick em- 

 bryos prior to the formation of intersomitic grooves, and stages 

 up to the formation of three intersomitic grooves constitute the 



ii 



1 Diagram to illustrate the blastodermal incisions in experiments of Types 

 I and II. Broken lines represent the incisions in Type I. Dotted line G to H 

 indicates the transverse incision in Type II in which incision E to F is omitted. 



2 Lateral view of the head-fragment of Type I pushed back to be severed 

 from the proamnion at point where a broken line intersects. 



material studied. It was found, on examination of the blasto- 

 derms that blood vessels were not generally to be found near 

 the embryonic axis at the time of the first intersomitic groove. 

 There are, however, sufficient exceptions to this to make an 

 examination of the blastoderms always desirable. Normal incu- 

 bation was generally allowed for about eighteen to twenty hours; 

 this had to be varied in different seasons. 



The Type I operation consisted in this: longitudinal incisions 

 were first made, separating laterally the anterior region of the 



