DE^^ELOPMENT OF WANDEEING MESENCHYMAL CELLS 535 



It is, therefore, difficult to state that the spindle-shaped cells 

 may not change to the heavier amoeboid pattern and vice versa. 

 But we shalisee that these two forms are the probable if not act- 

 ual forerunners of two groups of cells later, at any rate, with 

 permanent shapes and structures differing in much the same 

 way as the early appearances now differ. Figure 2 again shows 

 a sketch from life of wandering cells on the yolk-sac of a 48 hour 

 embryo, and here as usual the spindle cells are contrasted with 

 the amoeboid ones. The spindle cells are assumed to be future 

 vascular endothelial cells and the amoeboid cells are probably 

 the future chromatophores of the yolk-sac. 



The places from which cells wander out most actively are the 

 borders of the tail, and particularly from that mass of cells rep- 

 resenting the obliterated germ-ring. Figure 5 shows the cellular 

 arrangement around the tail end of a 48 hour embryo fixed and 

 cleared. The cells have withdrawn their processes. The open 

 space in the cell group at the tip of the tail, yk, is the place still 

 remaining between the borders of the germ-ring. Later, the 

 tail of the embryo grows over this cell group so that it is less 

 conspicuous. Figure 8, the tail end of a 56 hour embryo, shows 

 such a condition. 



The important fact which we shall later consider is that these 

 cells form a mass continuous with the mesenchymal cells within 

 the tail end of the embryo. The wandering cells may be inter- 

 preted to grow out from the end-bud or blastopore lip. They 

 are a scant rudiment of the peripheral or ventral mesenchyme 

 usually growing away from the blastopore lip over the yolk mass 

 in the reptile and the bird. It will be presently shown that 

 such an interpretation is upheld by the nature of the products to 

 which these wandering cells give rise. 



Figure 6 illustrates the wandering away of cells from the lateral 

 mesoblast of an embryo with two pairs of somites, 48 hours old. 

 Figure 7 shows the head end of a 56 hour embryo. Scarcely any 



Fig. 3 Camera outlines of wandering mesenchyme cells 48 hours old, all of 

 the future endothelial type, highly magnified. A and B are two outlines of the 

 same cell at a 6 minute interval. 



Fig. 4 Camera outlines of one cell drawn at 5 minute intervals A to E. The 

 cell is a migrating future chromatophore in an embryo 50 hours old (3b. DD ob.) 



