74 H. W. Rand and E. A. Boyden, 



It should be noted that our statements about the size and rate 

 of development of the eyes refer actually to the condition of the 

 pigmented portion of the eye. In the living worm the unpigmented 

 structures of the eye are so transparent as to be indistinguishable 

 from the transparent tissues in which they lie. It seems perfectly 

 safe, however, to assume that the size and configuration of the pig- 

 ment mass is a close index of the state of development of the eye. 

 The histogenesis of the planarian eye has been studied by Carrièee 

 (1882) and Jänichen (1896). Their work — particularly that of 

 JÄNICHEN — indicates that the development of the unpigmented eye 

 structures is contemporaneous with the appearance and characteristic 

 orientation of the pigmented cells. 



II. Additional evidence in favor of the conclusion that inequality 

 of the eyes is a fairly constant feature of regeneration from an 

 oblique anterior surface is afforded by the following experiment. 

 From ten individuals of Planaria maculata the head portion, inclu- 

 ding roughly one-fourth the bulk of the entire worm, was removed 

 by a cut making an angle of about forty-five degrees with the axis 

 of the worm. As in all cases of oblique cutting, there took place 

 a contraction of the tissues in the region of the wound in such a 

 way that the axis of the anterior portion of the decapitated worm 

 became concave upon the side from which the greater amount of 

 tissue had been removed (see Fig. E). New tissue developed in the 

 characteristic way. The condition of the regenerated eyes in the 

 ten cases was as follows. In two cases difference in the size of 

 the two eyes could be detected. Of the remaining eight cases, five 

 had distinctly unequal eyes, the larger eye being the one nearer 

 the convex edge of the piece, while three had a larger eye nearer 

 the concave edge of the piece. (We may conveniently designate as 

 the outer eye the one nearer the convex lateral edge of the piece, 

 and the other as the inner eye.) 



It has been noted that in the great majority of cases the larger 

 eye is oh the convex side of the curved axis of the worm. In three 

 animals of the group of ten last referred to, a second cut was made 

 one week after the first cut, the regenerating eyes being not yet 

 fully developed, with a view to changing the curvature of the axis. 

 This was done by cutting away a long longitudinal strip from the 

 longer lateral edge of the worm, as indicated in Fig. E, which 

 represents the worm regenerating after the first cut. The broken 



