PLATE I 



EXPLANATION OF FIGURES 



Figures 1, 6 to 11, inclusive, are from 9 mm. pig embryos fixed in Zenker- 

 formalin and stained with Giemsa. Figures 2 to 5, inclusive, are from 12 mm. 

 embryos fixed in Zenker-acetic and stained with borax carmine and Lyon's blue. 

 All figures are from sagittal sections, except figures 2 to 5 which are from trans- 

 verse sections of the aorta. The drawings, reduced one-fifth in reproduction, 

 were originally made with the magnifications obtained in the following combina- 

 tions of Zeiss apochromatic lenses and compensating oculars: 



Figs. 1 to 4, oc. 8, No. 3, obj. 



Fig. 5, oc. 4, 2 mm. obj. 



Figs. 6 to 11, oc. 6, 2 mm. obj. 



Recognition is due the artist, C. D. .Jarrett, for faitliful reproduction of cyto- 

 logical details. 



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