PLATE 2 



EXPLANATION OF FIGURES 



8 Normal embryonic stages 14 to 23. 



14 Spherical bilaminar blastocyst. 



15 Clear spot in medullary plate. 



16 First mesodermal cells forming cloud in the clear spot. 



17 Primitive streak with mesodermal crescents. 



18 Hensen's node. 



19 Primitive groove. Mesoderm still only beneath medullary plate. 



'20 Clear spot in front of Hensen 's node. Mesoderm extending beyond 

 medullary plate. 



21 Notochord equal to or shorter than primitive groove. 



22 Notochord longer than primitive groove. First somites forming. 



23 First coelomic rudiments. No heart. No optic cups. 



230 



