PLATE 



EXPLANATION OF FIGURES 



73 Embryo, ventral view, showing position of stomodaeum and the be- 

 ginning of the trochal folds (ectodermal proliferations). 



74 Same as figure 73, right side; the unshaded portion represents the trochal 

 prominence, especially near the ventral edge. 



75 A median longitudinal section of the preceding figure. 



76 A median longitudinal section of a twelve-hour embryo, to show the 

 early formation of the foot and the differentiation of the inner cell mass in the 

 formation of the digestive and reproductive systems. 



77 A longitudinal section to side of median line, to show further differen- 

 tiation. 



78 Ventral view of whole embryo to show formation of foot and trochal 

 folds. 



79 Longitudinal section of preceding figure; the lumen of stomach is formed. 



80 Horizontal section of figure 79, taken at a-a. 



81 Embryo, ventral view, to show the approximation of the two ends; the 

 lateral folds later become part of the trochal disc. 



82 Horizontal section of figure 79 taken at 6-6. 



83 Embryo from right ventral side. 



84 Same as the preceding, dorsal view, showing the condition of the ecto- 

 dermal cells; this figure represents a three-hundred-cell embryo. 



85 Embryo, ventral view, showing further differentiation of the foot and 

 development of the lateral trochal folds. 



86 Embryo, ventral view, showing the anterior and posterior folds of the 

 trochal disc; note position of mouth. 



87 Longitudinal section of embryo corresponding to figure 86. 



88 Horizontal section of figure 79 taken at c-c 



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