PRIMARY NEUROMERES AND HEAD SEGMENTATION 345 



Fig. 12 Frontal section through the open neural crests of an embryo, showing 

 irregular lobulations of crests and indefinite cell arrangement. 



Fig. 13 Frontal section through the developing brain vesicles of embryo, 

 showing the later neuromeres. 



Fig. 14 Cross-section of embryo at same state of development as figure 13, 

 showing mesodermal segmentation. 



