350 AIMEE S. VANNEMAN 



is beginning to give off a few mesoderm cells. These cells are 

 found in the neighborhood of each of the four embryonic areas, 

 and must either have migrated to these points along the blasto- 

 cyst entoderm, or else they are arising de novo from the yolk 

 sac entoderm at the point of contact of the latter with the em- 

 bryonic area. A further discussion of this point follows in the 

 conclusions. In any case, some of the cells are slightly amoe- 

 boid in shape and become more so in late secondary bud stages 

 when they appear to be traveling toward the ventral and cen- 

 tral portions of the future embryo in the posterior primitive 

 streak region (figs. 4 and 4a). In no. 290, some of the germ 

 cells lie in the space between the entoderm and the ectoderm 

 in a region between the embryonic areas as shown in text figure 

 2, a detail figure of the same being found in figure 3. Other 

 germ cells are to be seen in the embryonic areas. These lie close 

 upon the entoderm seemingly in the act of pushing their way 

 into the layer destined to become the gut entoderm (fig. 4). 

 A considerable number of divisions seem to occur among the germ 

 cells just prior to their entrance into the gut entoderm. In late 

 secondary bud stages where the diverticula are undergoing a 

 process of further elongation, conditions are similar to those 

 just described. Frequently, germ cells are found among meso- 

 derm cells which have budded out a distance from the primi- 

 tive streak. Specimen 226 shows such a condition (fig. 4). 

 The germ cells probably have no relation to the mesoderm cells, 

 but have only temporarily wandered among them. By the 

 time the stage represented in no. 276 (fig. 5a) is reached a condi- 

 tion in which the embryonic rudiments lie as 'slipper shaped' 

 structures each at the terminus of an elongated canal — the 

 germ cells are becoming fairly well established in the entoderm 

 of the embryos which are now in a relatively advanced primi- 

 tive streak stage. The position of the cells may be seen from an 

 examination of figure 5a. 



As the primitive streak advances, the entoderm previously 

 seen as a straight ribbon of cells, now commences to thicken and 

 push down and inward to form the intestinal groove. The germ 

 cells, which by this time have all migrated into this layer, are 



