DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 163 



space into which we have seen the free mesenchyme cells wander, 

 and finally within this space groups of these cells form the 

 yolk-sac vessels. 



Wenckebach ('86) described very clearly the origin of vessels 

 from the free mesenchyme within the segmentation cavity. My 

 study of a somewhat similar yolk-sac confirms the main points 

 brought out by Wenckebach and all serve as crucial facts in 

 support of the early theory advanced by Biitschli ('82) in his 

 "Die phylogenetischen Herleitung des Blutgefassapparates 

 der Metazoen." Biitschli held that in the Metazoa the lumen 

 of the blood vascular system was derived from the blastocoel. 

 Later, Hubrecht ('86) supported the same standpoint from his 

 studies on Nemertines. Hubrecht also found wandering cells 

 playing an important role. Ziegler ('87) gives a most careful 

 analysis of the continuity of the vascular lumen with the blasto- 

 coel in his studies on the development of the bony-fish. 



The foregoing description and figures of the origin of vessels 

 on the yolk-sac of Fundulus leaves no doubt that the vascular 

 lumen in these animals, coenogenetically at any rate, is con- 

 tinuous with the blastocoel or primary body cavity and is in 

 no way related to the coelom. 



Almost twenty years ago, Felix ('97) advanced the opposing 

 theory that the vascular lumen was really a localized or separate 

 part of the secondary body cavity, or true coelom. The many 

 decided objections to this theory from the standpoint of com- 

 parative anatomy, the presence of blood vessels before the 

 acquisition of a true coelom in the animal kindgom, and the 

 numerous embryological contradictions in its path were pointed 

 out in the discussion of this matter in the previous paper. 



Very recently Bremer ('14) has advocated the theory of the 

 origin of vessels as parts separated from the coelomic cavity, 

 or strands of cells from the coelomic epithelium. In the first 

 place, the material on which his investigation was based, early 

 human embryos, will scarcely permit such generalizations. 

 At least more suitable material could be found for the analysis 

 of this problem. Further than this, his consideration of the 

 questions involved does not lead one to form a definite idea of 



