196 Mary I s ah ell e Steele 



later formed nucleus of each row contributes to the formation of a 

 cone while the remaining nuclei form the retinulae. 



Unlike the rows of nuclei described by Kingsley and the spindle 

 shaped groups of cells described by Phillips the retinulae of the 

 regenerating eye do not appear to become separated into definite 

 groups before the development of the cones. In sections from 

 the same eye there may be groups of retinulae somewhat distinctly 

 separated from each other and other retinulae which constitute a 

 continuous band for a considerable distance. Figs. 60 and 61 

 illustrate these opposite cases. But even when groups can be 

 recognized there is no certain indication that a group belongs to a 

 single ommatidium. The group may contain a fewer or a greater 

 number of cells than belong to a single ommatidium. Besides 

 the retinulae continue to divide occasionally up to the time the 

 cones are diflferentiated. From the evidence furnished by a num- 

 ber of different specimens it appears that the definite separation 

 of the retinulae into groups does not take place until after the cones 

 are well advanced in their development. As the cones differen- 

 tiate from the periphery inward the retinulae become grouped 

 about them. As this grouping continues the retinular processes 

 become more and more slender, perhaps largely as a result of 

 mechanical pressure. 



The development of the cone as shown by my observations is the 

 result of intra-cellular secretion. In this respect it agrees with the 

 embiyonic development of the cones in the eye of the honey bee 

 as described by Phillips. It is directly opposed to the method 

 described by Patten ('87), Kingsley ('87) and Watase ('89), who 

 regard the cones as the result of the extra-cellular secretion of the 

 cone cells. 



The evidence furnished by the regenerating eyes of Palaemonetes, 

 Crangon and hermit crabs agrees with the observations of those 

 who do not find the cone and the rhabdom to be developed in the 

 embryonic eye as continuous structures. Some investigators 

 regard the rhabdom as merely an inward prolongation of the cone 

 cells. Kingsley finds such a relationship in the embryonic develop- 

 ment of the eye of Crangon. Patten regards the cone as extending 

 from the hypodermis to the basement membrane and as differen- 



