Inequality of the two Eyes in regenerating Planarians. 73 



two optic cups were formed upon the more heavily pigmented side, 

 both lying within one common clear region, which was slightly 

 larger than that containing the normal single eye of the other side. 

 Caeeière (1882) and Jänichen (1896) find double and supernumerary 

 eyes common in. planarians under both natural and experimental 

 conditions. Besides these six cases was observed a fragment of 

 worm (doubtless one of the detached slips) which was regenerating 

 a head, and here, too, the eyes were of conspicuously unequal size. 

 The mere fact that the two developing eyes in these more or 

 less irregular partially detached parts of the planarian are of 

 unequal size, or develop at unequal rates, may not in itself appear 

 to be of 3inj special moment. But the further observation that in 

 every one of the seven cases mentioned the larger eye was on 

 the convex side of the curved lateral slip indicates that 

 the condition must have some definite significance. Moegan (1901a) 

 describes inequality of the eyes in regeneration from an oblique sur- 

 face. He excised the middle portion of the body of Planaria lugubris, 

 the piece having an anterior cut edge oblique to the original axis 

 of the worm, and a transverse posterior cut edge. New tissue 

 appeared along the oblique edge and grew much more rapidly in 

 the more anterior region of that edge. A head was formed, not in 

 the middle of the cut edge, but well to one side and where the 

 formation of new tissue upon the cut edge had been most rapid. 

 At first "the head is small, and its median axis stands at right 

 angles to the cut surface. . . . The inner eye is sometimes less 

 developed than the outer, and this appears to be connected with 

 less development of the inner side of the new head . . ." (p. 198). 

 By "inner eye" Moegan evidently means the eye situated nearer 

 the more posterior portion of the oblique edge (see also Moegan, 

 1901b, p. 135). Baedeen (1902) made similar observations and gave 

 them a special interpretation. He says (p. 284) "There is an 

 interesting correlation between the lateral nerve cords and the eyes. 

 In oblique pieces the most direct stimulus to brain formation comes 

 from that nerve cord which is the more anteriorly situated. As a 

 rule the eye on the side corresponding to this nerve cord is deve- 

 loped earlier, and at first larger, than that upon the other. This 

 is still more marked in the regeneration of oblique cross pieces 

 divided in the median line, as shown by Moegan ... So, too, in 

 the regeneration of lateral slips the eye on the outer side is usually 

 earlier developed than that on the median". 



