DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 527 



germ layers were described actually as growing layers and from 

 this layer formation the different organs were built or produced 

 by foldings. Each cell was thought of as being passive and 

 only through its division was the formation of the organs brought 

 about. This initial investigation, and the only one so far as I 

 know, by young Wenckebach gave him an entirely different 

 view point regarding the processes of embryonic development. 



Wenckebach readily observed the cells of the mesoblast in- 

 dependently wandering in amoeboid fashion, often with extra- 

 ordinarily long protoplasmic processes, within the body of the 

 embryo as well as upon the hypoblast-free yolk. The wander- 

 ing cells move as with aim and purpose to form certain definite 

 organs. In the formation of the anlage and further development 

 of the heart, as well as the vessels and other structures, this 

 independent wandering of the mesoblast cells performs a most 

 important part. 



Unfortunately, Wenckebach's scientific efforts ceased and his 

 early study was unappreciated since on only one occasion was 

 it considered by an investigator, Raffaele ('92), who studied 

 similar material. Today the student gathers from text- and 

 hand-books as well as descriptive embryological contributions 

 much the same orthodox conception of the layers and foldings 

 as all important factors in the origin and development of embry- 

 onic organs. No doubt the growth of layers and folds does 

 contribute its part, but this part is almost negligible in a study 

 of the development of the vascular system and blood. 



Wenckebach and Raffaele, in their observations of the wander- 

 ing cells on the yolk-sac failed to recognize the erythroblast. 

 They were also unable to follow the processes in the differenti- 

 ation of pigment and endothelial cells in the way at present 

 possible with improved microscopes. The experimental embryos 

 without a circulation of the blood are also most instructive for 

 comparison in such a research. 



The wandering mesenchymal cells on the yolk-sac differentiate 

 into four distinct types of cells. The chromatophores of two 

 varieties, one black and one of a reddish brown color, endothelial 

 lining cells of the yolk vessels and islands of erythroblasts may 



