452 L. S. STONE 



mainly of nuclei with but little cytoplasm. The density increases 

 as one studies the sections farther ventrally. The few yolk gran- 

 ules which are present are similar in size to those of the smaller 

 ones in the mesoderm of the arches and many mitotic figures 

 can still be seen among the crest cells. The fusion of the median 

 ends of the first branchial groups with those of the hyoid in the 

 midventral line furnishes an illustration of the more compact 

 arrangement of the cells (fig. 34). 



As one follows the sections dorsally, the median ventral ex- 

 tremities of the branchial groups on each side are seen to approach 

 each other at successive levels. It is now quite obvious that the 

 branchial groups have taken on the form of the early branchial 

 cartilaginous skeleton. The dorsal extremities of the branchial 

 groups are continuous with loose crest cells, which possibly 

 linger behind to form connective tissue, although the rapid dif- 

 ferentiation of the surrounding mesoderm prevents any accurate 

 determination of this. Contiguous to the neural-crest aggrega- 

 tions the blood-vessel spaces can be seen permeating the meso- 

 derm of the arches, the circulation of blood in the mesoderm of 

 the arches having been established in stage 36-37. The crest 

 cells are more loosely arranged around the peripherj^ of the 

 mandibular arch — a possible indication that they are giving rise 

 to connective tissue. Over the maxillary process the crest cells 

 are followed with great difficulty owing to the fact that in front 

 of, as well as behind the stomadaeum there are many loose mesen- 

 chyme cells. The crest cells have already made their way into 

 the balancer, as has been shown by Harrison ('21). 



Placodes. Since the epibranchial placodes no longer appear in 

 the ectoderm, further description of the placodes will be confined 

 to the primordia of the lateral-line system (fig. 10). The supra- and 

 infra-orbital primordia extend farther ventrally, passing respec- 

 tively, around the anterior and posterior borders of the nasal organ. 

 They then pass medially in anticipation of the formation of the 

 sense organs on the upper jaw around the external nares. The 

 hyomandibular primordium has increased slightly in length and 

 passes to near the posteroventral border of the balancer. The 

 mandibular arid ventral hyomandibular primordia have extended 



