ORIGIN OF VASCULAR TISSUES 55 



ens in jungem Stadium vernichtet war, haben sioh doch auf der Seite, 

 wo die Area opaca unversehrt blieb, Gefiisse und ein Herz gebildet, was 

 niclit hiitte geschehen diirfen, wenn jene Ansicht, auf der die Hahn- 

 sclien Experiinente basieren, richtig Ware. 



Judging from his figure 27, one may well question whether the 

 'Narbe' has really excluded the area opaca. A line of separa- 

 tion wdthin the area pellucida has never in my experience yielded 

 such an enormously thick 'Narbe' as that shown in this figure. 

 The tissue on the extreme right of the figure is in all proba- 

 biUty a portion of the area opaca. Its failure to develop endo- 

 thehal cavities (and perhaps to absorb yolk) is difficult to ex- 

 plain. One never finds the splanchnopleuric mesoderm (myo- 

 cardium) in the heart region a single-layered structure unless 

 there is a pronounced oedema, so that the distension produced 

 by the coelomic fiuid enormously stretches the mesodermal walls. 

 This distension may furnish a mechanical explanation of the 

 failure of the cardiac endothelium to form on the side on which 

 the oedematous condition is exhibited. Certain it is that endo- 

 cardial tissue had little space in which to develop under such 

 conditions. The space between splanchnopleure and entoderm 

 is completely occluded; in other words, if this explanation be 

 correct, endothelium w^as potentially able to develop if the 

 mechanical relations of the coelom had not prevented it. One 

 could also assume that those conditions w^hich produced the 

 abnormal coelom had likewise injured the tissue which would 

 have produced prevascular anlagen, and that both abnormal 

 conditions resulted from the same cause. 



Hahn (17, pp. 410-413) has also commented on the conditions 

 in Griiper's embryo 'C'. He points out the fact that in '' Graper's 

 embryo 'A' the heart is also unilateral. He states (p. 410) : 



Die Tatsache, dass cine der operierten Seite entsprechende Herzhalfte 

 bei Embryo A nicht zur Entwicklung gelangt ist, begrlindet nun Graper 

 damit, dass dieselbe 'eben nicht notig war.' Er fiihrt dies dann noch 

 weiter aus mit den Worten : 'Auf der operierten Seite wurde die Aniage 

 der Gefiisse durch den Eingriff verzogert (also nicht total verhindert!), 

 so dass die andre Seite Zeit hatte, eine Herzhalfte zu bilden, die dem 

 ganzen Herzen gleichwertig war; numehr war die Bildung des zweiten 

 Herzens unnotig und unterblieb.' 



