DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 169 



developmental environment. Therefore, the four distinct mesen- 

 chymal anlagen each give rise to a perfectly typical and distinct 

 cell type although all develop in, as far as one can judge, an 

 identical environment, the cavity of the yolk-sac between the 

 ectoderm and the periblastic syncytium. The differences among 

 the four cell types produced are from the standpoint of our 

 present knowledge in all probability due to the potential differ- 

 ences among the apparently similar mesenchymal cells from 

 which they arose. The four types including endothelial cells 

 and erythrocytes we must consider from an embryological stand- 

 point as arising from different mesenchymal anlagen. 



SUMMARY 



The yolk-sac of the teleost egg is a most beautiful object for 

 observing the movements and migrations of cells hi the develop- 

 ing embryo. Such a yolk-sac has only one really definite con- 

 tinuous membranous cell layer, the ectoderm ; a true endodermal 

 layer is absent, though a superficial syncytium, the periblast, 

 fuses with the actual yolk surface. The mesodermal layer is 

 represented by numerous separate wandering mesenchymal 

 cells. These free\y wandering mesenchymal cells may be clearly 

 observed through the perfectly transparent ectoderm as they 

 move over the surface of the periblast. 



The present contribution attempts to give a full account of 

 the movements of the mesenchyme cells and their manner of 

 development and differentiation in the yolk-sac. Observations 

 have been made on the normal embryos from the earliest stages 

 at which the mesenchyme wanders out upon the yolk up to the 

 late embryo in which a complex vitelline circulation is fully 

 established, and all of the products of the yolk mesenchyme 

 completely differentiated. The study has been greatly facili- 

 tated by a comparison of the normal embryos with specimens 

 in which the circulation of the blood was experimentally pre- 

 vented from taking place. In such specimens the cells on the 

 yolk-sac never became confused or contaminated with other 

 cellular elements introduced by the circulating blood. The 



