PLATE 1 



EXPLANATION OF FIGURES 



1 3.5-mm. Chelydra serpentina (10 segments). Section passing through head 

 region. X 175. 



2 Same series. Section passing transversely through hindbrain vesicle. 

 X 175. 



3 Same series. Section passing transversely through tip of forebrain. X 175. 



4 4-mm. Chelydra serpentina (13 segments). Sagittal section in median 

 plane through prechordal plate. X 175. 



5 Same series. Sagittal section laterad of neural tube, through the N. tri- 

 geminus and second head somite. X 175. 



166 



