302 CHARLES R. STOCKARD 



origin. This is left to the imagination of the individual investi- 

 gator and further possibilities of error are open. 



Wenckebach's ('86) observations of living embryos are most 

 important in this connection. He noted that not only the layers 

 but that independent mesoblast cells with amoeboid processes 

 wander out of the embryo and over the yolk. These wandering 

 cells play a great part in the formation of the anlage of the heart 

 endothelium and great vessels. In the Teleost embryo one may 

 readily observe these wandering cells in the yolk-sac, and they 

 doubtless give rise to the yolk vessels and blood islands as well 

 as the pigment cells so abundantly present. 



Ziegler ('87) has suggested that it may be that the blood 

 anlage in phylogeny has been passed to the mesoderm from 

 the endoderm, and for this reason the endodermal origin may 

 sometimes occur in coenogenetic development. Goette ('90) 

 also held that the endodermal origin of the blood was the more 

 primitive one. This point of view overlooks the fact that in 

 the invertebrates generally the blood and vessel walls are de- 

 rived from the mesoderm. 



In discussing the question of the place of origin of the vessels, 

 Felix ('97) points out that Riickert ('88) claimed in Selachians, 

 that the aorta arose in loco. P. Mayer and Strahl ('95), have 

 also stated that the great vessels are late in appearing and arise 

 in loco in the embiyo's body. Felix states that the glomerulus 

 of the bird mesonephros originates in loco independently of the 

 aorta. Further that the stammvene, venenplexus of the mesone- 

 phros, certain vessels of the glomerulus, and also the mesenteric 

 artery along with the aorta in the Salmoniden arise in loco. 

 Regarding the anlage of the heart and vena sub-intestinales, 

 Felix is not certain but thinks that these likewise arise in loco. 

 All of these observations are directly opposed to the theory 

 of ingrowth of vessels from the yolk-sac, the parablast theory 

 of His ('75) as well as the outgrowth of vessels in the sense ad- 

 vocated by Sobotta ('02). 



Ziegler ('89) and Felix ('97) have both speculated considerably 

 as to the relationship of the cavity of the circulatory system with 

 the primary body cavity and the coelom. Ziegler pointed out 



