50 FRANKLIN P. REAGAN 



mine experimentally whether primitive-streak mesoderm or para- 

 blast was the source of the vascular tissue; this he tried by pre- 

 venting the development of the primitive streak. He was un- 

 able to obtain decisive results. Uskow (72) likewise attempted 

 to arrest the development of the mesoderm, but was unsuccessful. 

 The work of Graper and Hahn along these same lines furnishes 

 a foundation for the subsequent experimental work on vasculo- 

 genesis. Their observations have, unfortunately, received little 

 attention from later investigators; for this reason it seems 

 advisable to consider rather minutely their methods and results. 



Graper' s work 



Graper 's investigation (16) is primarily an investigation of the 

 process of heart-formation, in which the main purpose is to show 

 that the heart, in accordance with the 'Rablsche Erklarung,' is 

 essentially a fused pair of vitelline veins, and its efferent vessels 

 are formed in genetic continuity with its own endothelium. 

 The work attempts to show the explicability of double and 

 multiple hearts on the basis of a failure of the mesial fusion of 

 the endothehal tubes, which originally were laterally located. 

 Evidently Graper regarded our knowledge of heart formation as 

 somewhat uncertain and theoretical. A second purpose of 

 Graper's work, which was not essentially experimental, was to 

 prove that the earhest vascular tissues on the yolk-sac are of 

 entodermal origin; that they are formed from entodermal cells 

 which separate from that layer, invade the germ- wall, and 

 emerge to form endothelial cells and blood-cells; that at least 

 some of the invading cells become 'Dottertrager' which partici- 

 pate in the formation of vascular tissues and in their nourish- 

 ment. The work attempts to disprove the view of Riickert 

 and MoUier that the vascular tissue is derived from primitive- 

 streak mesoderm which is proliferated into or migrates into the 

 vascular area and possibly into the germ-wall. 



Ordinarily when blood-islands are first recognizable, they are 

 found lymg between the mesoderm and the germ-wall. As 

 Riickert has observed, their position when first recognizable 



