454 



L. S. STONE 



A study of the early branchial skeleton up to this stage shows 

 conclusively that so far the cartilages have their origin in the 

 neural crest. However, in the case of the second basibranchial, 

 which is the last cartilage of the visceral skeleton to appear, stage 

 42 shows more convincingly than any of the previous stages that 

 this cartilage is of mesodermal origin. The caudal extremity of 



35 



Fig. 35 Frontal section through mesoderm of visceral arches at stage 39, 

 showing positions of their procartilages. X 37. 



Fig. 36 Frontal section through the cartilages of the arches at stage 42 when 

 true cartilage is laid down, showing the same positions of the cartilages occupied 

 by aggregations of crest cells. X 37. 



the first basibranchial is attached to mesoderm which continues 

 ventrally toward the pharynx. Out of this mesoderm along the 

 anterior part of the pericardial chamber is formed the second 

 basibranchial. Its distal half lies ventral to the level of the other 

 cartilages. The mesoderm continuous with its distal extremity 

 is the anlage of the thoracicohyoideus muscles, while the meso- 

 derm continuous with its proximal or attached extremity is the 

 anlage of the geniohyoid muscles. It has not developed cartilage 



