52 FRANKLIN P. REAGAN 



valeiir est nulle, mais qui prouvent la theorie de Torigine ento- 

 dermo-parablastiqiie des vaisseaux sanguins." (Parablaste here 

 refers to germ-wall.) If this case reported by Jan Tur be of 

 'significance, one might, in following Graper's line of reasoning, 

 be compelled to enunciate a new germ-layer. Perhaps a most 

 feasible explanation of these isolated blood-islands is the assump- 

 tion that we are here dealing with migrating mesodermal (mesen- 

 chymal) cells; such cells may perhaps arise either from meso- 

 derm, entoderm, or both. Once such a cell has arisen from 

 entoderm or mesoderm, it might well be designated as meso- 

 derm or mesenchyme. As will be considered later, the work of 

 Wenckebach, Raffeale, Stockard, and the waiter has shown, the 

 teleost yolk-sac, entirely devoid of true entoderm, furnishes an 

 excellent place in which to study the actual migration of living 

 mesenchyme cells and theii' transformation into vascular tissue. 

 A study of such migrating cells might convince one that the 

 traversing of a distance of 1 mm. by mesenchyme cells would 

 not be out of the question in case of Graper's isolated blood- 

 islands. 



Riickert points out the possibility that even though the cells 

 emerging from the germ-wall may be closely approximated to 

 the entoderm, they may conceivably have come from mesoderm. 

 Graper answers this by maintaining that ultimately all the meso- 

 derm in question came from entoderm (which is certainly not 

 true in case of primitive-streak mesoderm in the chick) ; Graper 

 also maintains (p. 381) that even if the cells in question had 

 come from mesoderm, they should necessarily be regarded as 

 entoderm when they were in association with that Isbjer, "so 

 konnte man doch unmoghch von mesodermaler Bildungsweise 

 reden." Such confusion at least shows the need of a definite 

 terminology. 



van der Stricht (67) described yolk-laden cells in the meso- 

 derm, and beUeved that such cells had taken on a yolk-content 

 by means of protoplasmic processes which reached down into 

 the yolk. This offers a possible explanation of the "Dotter- 

 trager" of many authors. In some cases such protoplasmic 

 processes might not be preserved in fixed material. 



