450 



L. S. STONE 



may be seen many mitotic figures indicating that there still is a 

 rapid increase in the number of cells as well as a migration. Up 

 to this point the crest cells have shown characteristic fine yolk 

 granules in their cytoplasm which was an aid in differentiating 

 them from the surrounding tissue. Now the yolk granules have 

 become very small or have been largely absorbed within the crest- 

 cell aggregations. In many places the mesoderm of the branchial 



Figs. 33 and 34 Frontal sections, showing at stage 36 and 37-38, respectively, 

 large numbers of crest cells at median and posterior borders of mesoderm of man- 

 dibular arch and fusion in the midline of crest cells of hyoid and first branchial 

 groups to form the first basibranchium. X 37. 



arches is being hollowed out by the formation of developing blood 

 vessels. 



If the sections are followed ventrally it can be seen that the 

 crest cells of the hyoid arches unite in the midventral line in front 

 of the pericardium. The first branchial group, after it has united 

 with the second branchial group (fig. 33), joins the crest cells of 

 the first branchial group and from the opposite side attempts 

 farther ventrally to continue with the posterior border of the 

 fusion of the hyoid crest cells. The third and fourth branchial 

 crest-cell groups have not yet joined at their ventral extremities. 



