170 CHARLES R. STOCKARD 



wandering cells may thus be completely followed through all 

 stages in their isolated position. 



The behavior of the migrating cells impresses one with the 

 very important role of such elements in the formation of tissues 

 and organs, particularly the blood vessels and certain blood 

 cells. The observer is also struck by the fact that such phenomena 

 are extremely difficult if not actually impossible to interpret 

 from a mere study of dead specimens cut in serial sections. Of 

 course the study of sections greatly aids the observations on the 

 living, and but for the fact of a long acquaintance with the 

 Fundulus yolk-sac in sections, the writer would have found it 

 much more difficult to identify many of the cells in life. 



The results of this investigation of wandering mesenchymal 

 cells may be summarized as follows : 



1. The wandering cells begin to migrate away from the embry- 

 onic shield or line of the embryonic body at an early period, when 

 the embryo is about 40 hours old, the germ ring having almost 

 completely passed over the yolk sphere to enclose its vegetal 

 pole. The cells migrate away chiefly from the caudal end of the 

 embryo, only a few wandering oui/ ^om the head region. The 

 regions of the yolk-sac thus suggest an area opaca about the tail 

 end and an area pallucida around the neighborhood of the head. 



All of the cells wander into the so-called subgerminal cavity, 

 the space Wilson ('90) and others consider a late stage of the 

 segmentation cavity, between the yolk-sac ectoderm and the 

 periblast syncytium. 



When the cells first appear they are all closely similar in shape 

 and about the same size. Very soon, however, they begin to 

 exhibit certain differences. Many become elongate spindle 

 cells with delicate filamentous processes, sometimes producing a 

 stellate appearance. Others are more amoeboid in shape with 

 conical pseudopod-like processes which are constantly being 

 thrown out at one place and withdrawn at another. Still a 

 third class of cells appear somewhat later than the other two; 

 these are more circular in outline with short thick pseudopods 

 and are more slowly moving. 



