DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 537 



cell migration is taking place from this region. A few mesenchyme 

 cells are found along the border of the head; these cells later 

 take part in either the formation of the heart or pericardial wall. 

 The tail end of the same embryo, figure 8, shows a remarkable 

 contrast; here there is an enormous wandering out of cells from 

 the mesoblast of the embryo. The two figures show the huge 

 periblast nuclei to be widely distributed throughout the surface 

 of the yolk sphere. These drawings are from cleared specimens 

 and the cell outlines are more or less circular without the beauti- 

 ful processes characteristic of the living. 



The tail end of a living embryo 72 hours old, some time before 

 the blood began to circulate, is illustrated by figure 9. The 

 circle beneath the tail represents Kupffer's vesicle. The various 

 shaped mesenchymal cells are represented in the act of wander- 

 ing out over the nearby surface of the yolk. The embryo and 

 yolk are beautifully transparent in life and the cells are clearly 

 seen as they move upon the surface of the periblast. 



An entire embryo, except the anterior portion of the head which 

 extends beyond the curve of the yolk, is shown in figure 10 at a 

 lower magnification. This specimen was 76 hours old when 

 drawn. The heart had begun to contract slowly and feebly 

 but no circulation of fluid had begun. Groups of mesenchymal 

 cells are seen wandering away from the lateral and particularly 

 the caudal regions of the embryo and are now scattered broadly 

 over the yolk surface; there being very few, however, in the 

 anterior region. The lateral plates of the mesoderm are seen 

 at the sides of the head, and a circle at the caudal end indicates 

 the Kupffer's vesicle which is always clearly shown at this stage. 



In embryos of 72 hours, and somewhat earlier, there are 

 wandering out from the tail region a niunber of cells slightly 

 smaller than the two types mentioned above. These small 

 cells tend'to be more or less circular in outline but show slow 

 amoeboid movement as they send out short blunt processes. 

 They group themselves into small clumps and are to give rise to 

 erythroblasts or future red blood corpuscles in the yolk-sac as 

 shall be discussed beyond. Figure 31, page 569, shows six 

 such cells from the living yolk-sac of an embryo 90 hours old; 



