ORIGIN OF VASCULAR TISSUES 51 



gives little clue to their ultimate origin. Graper points out that 

 Riickert's study of avian blood-islands was confined largely to 

 the posterior portion of the blastoderm, where he (Graper) 

 believes the picture is not clearly marked; his own studies deal' 

 mostly with the anterior portion of chick blastoderms in which 

 the head-process has a length of about one millimeter. In this 

 location he was able to find isolated blood-islands in a region 

 not yet invaded by laminated mesoderm. He sometimes found 

 blood-islands more than a millimeter distant from any meso- 

 derm. Such blood-islands lay between germ- wall entoderm and 

 ectoderm. Relying upon the extreme improbability that their 

 origin should ever be regarded as ectodermal, he believes that 

 such blood-islands furnish conclusive evidence for the entodermal 

 origin of the vascular tissues. 



Riickert (Hertwig's Handbuch, Bd. 1, S. 1180) writes: 



Worin besteht aber die Bedeutung des Dotters fiir die Blutbildung 

 der Wirbeltiere? Auch hier gibt das Ei des Platydadylus einen Finger- 

 zeig. Da die Verdickung des unteren Blattes erst nach dem Erscheinen 

 der Blutanlagen auftritt bezw. (vorn) zu voller Ausbildung gelangt, so 

 kann dieselbe nicht die Aufgabe haben, den Entoblast zur Abgabe von 

 Zellenmaterial an die Blutinsel vorzubereiten. Das Wesen des Vor- 

 ganges kann also nicht in einer Zellen sondern nur in einer Stoffabgabe 

 des Dotters an die Blutanlagen beruhen. 



Graper dismisses this observation of Riickert's with a state- 

 ment that it apparently rests ''auf Grund vergleichender Forsch- 

 ungen." If it be true, as in the instance cited by Riickert, that 

 blood-islands may develop in a region where there is as yet no 

 thickened entoderm, the isolated blood-islands of Graper are not 

 all-significant, albeit they display a condition of extreme interest. 

 If blood-islands can develop at a distance from mesoderm, and 

 at a distance from thickened entoderm in another, it is evident 

 that no valid generalization can be made concerning either of 

 these layers as the sole source of the vascular tissue. 



Jan Tur (71) described vascular anlagen completely isolated 

 from entoderm, mesoderm, and from ectoderm. Yet he stated: 



"Les elements de ce 'parablaste accesoire' qu'on pourrait 

 designer sous le nom de 'parablaste sous-germiaale' peuvent 

 donner naissance a de vraies formations vasculaires dont la 



