DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 539 



a circulation was partially established in this specimen but 

 these cells had not yet been taken into the vessels. 



The various wandering cells then represent the mesodermal 

 layer of the yolk-sac in the teleost. They never assume a 

 membranous layer-like arrangement, but finally differentiate 

 into the characteristic structures of the yolk-sac. As is shown 

 by the illustrations, these cells are very numerous and during 

 their earlier stages are actively changing their shape and mov- 

 ing over the yolk surface. 



We may now consider the further development of such cells 

 the living embryos. 



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Fig. 7 Outline of the head end of a 56 hour embryo, scarcely any wandering 

 mesenchymal cells in this region. Large periblast nuclei scattered over yolk 

 surface. 



Fig. 8 The caudal end of the same embryo; note the great contrast- in the 

 abundance of out-wandering mesenchymal cells. Glycerine-formalin specimen. 



Fig. 9 The caudal end of a living normal embryo of 72 hours, with beauti- 

 fully delicate mesenchymal cells wandering away from the body; Kv., Kupffer's 

 vesicle (3b. 2/.3 ob.). 



