DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 567 



The arrangement of these wandering cells in the yolk-sac, 

 figures 7, 8 and 10, suggests in a way the regions of growth of the 

 yolk-sac mesoderm in other meroblastic eggs. About the caudal 

 region there is an 'area opaca' formed by the great number of 

 wandering cells, while around the head end the scarcity of 

 mesenchymal cells might be considered an area pallucida. 



All of the yolk-sac blood islands in the Fundulus embryos are 

 formed from certain of the early wandering mesenchymal cells 

 on the yolk. During the early wandering stages when the future 

 endothelial cells have a spindle shape and the future chromato- 

 phores are large amoeboid cells, other mesenchymal cells on the 

 yolk are small and more or less circular in outline. These small 

 circular cells move slower than the other types and throw out 

 short thick pseudopod-like processes. 



Whereas the spindle shape cells wander away from the embryo 

 along its entire lateral border as well as the caudal end, and the 

 large amoeboid future chromatophores have almost an equally 

 extensive place of origin, the small round cells wander out only 

 from a limited region. The earliest ones of these to be seen are 

 near the caudal end of the embryo before the tail fold has com- 

 pletely separated the caudal end from the yolk surface. As the 

 tail is moulded free from the yolk-sac, the point of outwandering 

 of the circular cells follows the place of union between the ven- 

 tral wall of the tail and the yolk-sac. Just at this place the 

 mesenchyme of the embryonic body extends itself out on to the 

 yolk as free wandering cells. 



In a study of sections this mesenchyme is found to lead directly 

 to the end-bud, Endknospe, which may be considered to represent 

 the blastopore lip. The cord of mesoblast which has been des- 

 ignated as intermediate cell mass leads caudally to the end-bud 

 which is well out in the tail. The ventral cells from this mass 

 wander away into the yolk-sac from the extreme caudal position 

 and other cells also wander away laterally from the intermediate 

 cell mass. Figure 8, the tail end of an embryo 56 hours old, 

 illustrates very well the place of outwandering of the round cells. 

 Few, if any such cells wander out from the lateral borders of the 

 embryo in regions more cephalad than this. 



