202 Mary Isahelle Steele 



removed by the operation, that regeneration would have followed 

 more rapidly. An inference which is'supported by the observa- 

 tions of Zeleny ('05), who finds that in regenerating appendages of 

 crayfish, an increase of the injury, increases the rate of regener- 

 ation. On the other hand, the same inference cannot be applied 

 to the regenerating eyes of Palaemonetes; for as has already been 

 pointed out for this form new ommatidia differentiate more 

 rapidly when a part of the old ommatidia remain uninjured. 



5 CoTTipanson of Normal and Regenerated Eyes 



It seldom happens that a regenerated eye appears altogether 

 normal either from external or internal examination. Externally 

 they frequently appear abnormal in shape and are always smaller 

 than the opposite eye. These external abnormalities are, however, 

 of no especial importance except in so far as they may indicate 

 internal conditions. A common external feature which is sugges- 

 tive of internal conditions is the irregular arrangement of the pig- 

 ment masses. Internal examination shows that these masses are 

 frequently remains of broken down retinula?. 



Besides the pigment remains of the old retinulse, a number of 

 other abnormalities may appear which make it difficult to interpret 

 sections correctly. The arrangement of the retinulae makes it 

 difficult to group them into distal and proximal rows of nuclei as 

 can be done readily in the normal eye. It is quite possible in cross 

 sections to select ommatiia in which eight retinulae, the typical 

 number, can be counted but in other cases this number cannot be 

 recognized owing possibly to the suppression of the eighth retinular 

 cell, which is rudimentary in the normal eye. The diff'erence in 

 the length of normal and regenerated ommatidia is quite noticeable 

 in many cases. The regenerated ommatidia are often much 

 shorter. The new ommatidia might have grown to normal size, 

 however, had the experiment covered a longer period of time. 



The remaining significant diff'erence between the normal and 

 regenerated eyes is that in some regenerated eyes the optic ganglion 

 is not complete. This diff'erence appears only in the hermit crabs 

 as these forms may develop an eye after half the optic ganglion has 



