ANALYSIS OF FORM REGULATION. 1 69 



associated with the unpigmented area representing the sensory 

 cells in each case. Frequently, however, in the four-eyed indi- 

 viduals the unpigrriented areas as seen in dorsal view in the living 

 animals are continuous between the anterior and posterior eye on 

 each side. But the pigment spots are, so far as my observations 

 go, always distinct and arise separately, not by division of a pig- 

 ment mass already present. An attempt to consider the possible 

 factors involved in these peculiar phenomena must be postponed 

 until the data are presented in full. It may be said, however, 

 that these cases of supplementary eye-formation suggest that the 

 formation of a single eye or of a pair under normal conditions 

 inhibits the formation of further eyes within a certain region of 

 the head, while in anaesthetic media this correlative inhibiting 

 effect is not sufficient to prevent the formation of new eyes as 

 the animal becomes more and more fully acclimated and the head 

 grows larger. 



The relation between the formation of single median eyes or 

 paired eyes and the length of the piece in anaesthetics and in 

 water demonstrate very clearly that even the localization of such 

 an organ as the eye-spot is dependent, not merely on the charac- 

 ter of the tissues from which it is formed, but upon the organiza- 

 tion of the whole piece of which it forms a part. Moreover, the 

 fact that, other things being equal, a longer piece, is necessary 

 for the formation of paired eyes in the anaesthetic than in water 

 indicates that the conditions or processes in other parts of the 

 piece are less effective in the localization of the eyes in anaes- 

 thetic media than in water. 



VII. The Effect on the Whole Animal. 

 ' When whole animals of large size (15—18 mm.) are placed in 

 the anaesthetics the first effect besides the more or less complete 

 cessation of locomotion is usually the loss of the pharynx, which 

 is often extruded within forty-eight hours, though it may be 

 retained and undergo degeneration in situ. Following its extru- 

 sion or degeneration a small new pharynx is usually slowly 

 formed in the old pharyngeal pouch, but this never attains 

 anything like the size of the original organ. Small young 

 animals usually do not extrude the pharynx, and, so far as I 

 have been able to determine, it does not degenerate when retained. 



