Regeneration m Compound Eyes of Crustacea 205 



abnormal as it may be. The pigment shown in this figure repre- 

 sents the maximum amount seen in any section. For the most 

 part it presents no definite arrangement but hes in irregular 

 masses clustered within a fairly well defined area. In a few 

 sections, however, a part of the pigment shows a tendency toward 

 a normal arrangement as if the pigment granules were contained 

 within the processes of the retinular cells, and rudimentary 

 ommatidia can be recognized (Fig. 59). 



Any attempt to explain the phenomena presented by the eye 

 under discussion may appear somewhat premature, since it pre- 

 sents a practically unique case so that but little data for comparison 

 is available. In the first place this is the only well established 

 case of any attempt of Palaemonetes to regenerate ommatidial 

 structures after the removal of a large part of the optic ganglion. 

 It seldom happens that any regeneration takes place from the eye 

 stump of Palaemonetes when no more than half of the optic gan- 

 glion remains. Before attempting to explain the phenomena, 

 therefore, it is well perhaps to examine the results of other observa- 

 tions that may suggest an explanation. 



From the evidence obtained from the study of normally regener- 

 ating eyes the indications are that the first new regenerated tissue 

 is largely of an indiflFerent character, i. e.^ capable of giving rise to 

 different structures, as determined by conditions more or less 

 external to itself. It has been seen that in the regenerating eye 

 the primary hypodermis gives rise to the cells which develop into 

 the diff^erent structures of the ommatidia. In the earliest begin- 

 ning of diflPerentiation, if a cell divides so that the plane of division 

 is at right angles to the surface the two resulting cells are hypo- 

 dermal cells. If on the other hand the plane of division is parallel 

 to the surface the inner one of the pair thus formed becomes a 

 retinular nucleus and the outer one remains hypodermal in char- 

 acter. At this stage the only apparent difference between the two 

 nuclei is in their respective positions. In later cell generations 

 when the division plane is parallel to the surface the inner nuclei 

 of the pairs formed become crystalline cone nuclei. Thus we have 

 cone nuclei, retinular nuclei and 'hypodermal nuclei indistinguish- 

 able except for their relative positions. Apparently the subsequent 



