Fig. 2. Ä, B, C, D, E. Sagittal sections of planarian embryos at the period when 

 the sphere is becoming flattened as the embryo assumes the definitive shaj^e. In each 

 figure the central clear space represents the area occupied by the mass of distinte- 

 grating yolk cells which have passed in by way of the embiyo-pharynx. The outlines 

 of the stippled area show the extent in different regions of the surrounding wall in 

 which are contained the cells descended from the blastomeres. In each figure the 

 anterior end is to the left and the future ventral side is the lower one. The thickening 

 of the antero -ventral region of the wall marks the place at which the brain will later 

 appear. The long axis of the embryo-pharynx is shown in each by the arrow marked 

 pit' and 2X pii the anläge of the permanent pharynx is shown or its position indicated. 

 A, Copy of Bardeen's figure of a P. maculata embryo. B, Copy of Ij im a' s figure 

 of D. lacteum. C, My own figure of P. maculata reversed for the sake of more easy 

 comparison w4th the others of this series. Z), Represents the outline of a planarian 

 embryo at this stage, and is inserted to show the condition which Matties en de- 

 scribes in P. torva. The outlines of this figure are not taken from any one of his figures 

 for this stage would be about mid-way between his figures 72 and 73 a. E, Shows 

 the condition found by Stevens in P. simplicissima and is from the figure 45 of 

 this author. The dotted line by which the outline is completed represents the portion 

 of the embryo which is omitted from Stevens figure. F, Is a diagram of the spheri- 

 cal embryo cut in the long axis of the embryonic pharynx. 



