156 MARY J. GUTHRIE AND HOPE HIBBARD. 



KXI'I.ANATION OF PLATE IV. 



FIG. i8c. Third section in the same series as Figs. 180 and i8& showing more 

 numerous and compact mesenchyme cells. 



FIG. 19. Section through slightly older embryo showing continued multiplica- 

 tion of mesenchyme cells. 



FIG. 20. Section through a later stage than Fig. 19. 



FIG. 21. Vertical section through an early gastrula. 



FIG. 22. Horizontal section of same stage as Fig. 21. 



FIG. 23. Vertical section of older gastrula. 



FIG. 24. Entire living gastrula. 



