DEVELOPMENT OF WANDERING MESENCHYMAL CELLS 153 



flowing. The taking up of the islands by the circulation is most 

 interesting to observe. At first those cells near the tail of the 

 embryo, which are enclosed by endothelium, are taken. A few 

 of the corpuscles are shaken by the current and these strike 

 against the other members of the group until all become loosened 

 and move slightly to and fro; finally one or two are suddenly 

 washed away, then others follow few at first, until the entire 

 group loses its stand and is swept away by the current. One 

 and then another of the islands may be seen to perish in this 

 manner before the irresistible force of the tiny stream. 



Yet even after the yolk circulation is fairly well established 

 a number of islands of the round cells may still exist unsurrounded 

 by the endothelial vessels. Figure 32 shows such a case. A 

 well established current flows through the vessel to the right, 

 while to the left is an incipient vessel not yet connected with the 

 current. In the center of the figure is a group of corpuscles 

 below a huge black chromatophore. These round cells con- 

 stitute a blood island still unenclosed by vessel endothelium; 

 in the course of a few hours, however, they too will become 

 enclosed by a vessel and subsequently be included among the 

 circulating blood cells. 



The early erythroblasts which are in this manner included 

 within the circulation assume a circular outline as they float in 

 the current. In the previous paper, figures 31 and 32 of cross 

 sections through the intermediate cell mass, and figures 34 and 

 35 of cells in a yolk island, all from a young 72 hour embryo, 

 illustrate the definite circular outline of these cells. In life they 

 may be carefully observed in the small vessels where a single 

 cell passes with difficulty, and are here seen to be globular in 

 shape and to retain their slight amoeboid movement. The 

 form of the early erythroblast is readily changeable for the first 

 one or two days after entering the current. After two or three 

 days, that is, in embryos five or six days old, the cells in the blood 

 current assume a typical erythrocyte appearance, becoming 

 elongate and elliptical in shape when seen from one position, 

 while they are thin in profile view. They are now ellipsoidal 

 nucleated red blood corpuscles. At about the time they begin 



