1905.] NATURAL SCIENCES OF PHILADELPHIA. 143 



the corneal pigment cells acquire pigment. The spindle shape of the 

 retinula so marked in the larval condition is still retained, the retinula 

 being widest at about one-third of the distance from the cone cells to 

 the basement membrane. The relatively large nuclei of the retinula at 

 this time are near together, and in no definite arrangement in the thick- 

 est portion of the cell group. The cytoplasm of the cells is uniform 

 except as described for the distal end, and the cell membranes between 

 the various cells are not visible. The outside boundaries of the 

 retinula group at this time and all through development mark off the 

 retinula from its surrounding pigment cells very sharply, and the 

 difference in the appearance of the protoplasm makes it impossible to 

 confuse the various cells. 



The portion of the retinula which lies between its thickest part and 

 the basement membrane is a strand of protoplasm circular in cross- 

 section and without any signs of differentiation. As the basement 

 membrane changes its position, by a process to be described later, 

 coming to lie near the optic ganglion, this portion of the retinula be- 

 comes longer, and the changes which take place in the retina consist 

 of the making over of this strand of protoplasm into the retinula cells 

 proper. This change progresses proximally and consists in the widen- 

 ing out of the cells with its accompanying rhabdome formation. The 

 nuclei shift as the retinula enlarges and elongates until we reach a con- 

 dition (fig. 2) in which two of them are at one level and the other six 

 (or seven) are at a lower level and arranged in a rosette. 



At the time when the nuclei are arranged in this manner, the most 

 distal portion of the retinul,a becomes arranged in a definite rosette, 

 caused by each of the cells forming a projection which shows its dis- 

 tinctness from the others in the group in cross-section. This arrange- 

 ment also progresses proximally until in the adult condition it is found 

 throughout the length of the retinula. At the same time the inner 

 portion of the mass becomes still more differentiated, and in the stage 

 just mentioned the axis of the distal end is occupied by a strand of 

 protoplasm which takes the iron haematoxylin stain (the future rhab- 

 dome) surrounded by a clearer protoplasm. Outside of this clear area 

 the protoplasm is granular and pigment deposition takes place here, 

 keeping pace with the inner differentiations, and these changes also 

 progress toward the basement membrane. The rhabdome formation 

 precedes slightly the formation of the clear protoplasm around it, and 

 the proximal end of the forming rhabdome shades off gradually into 

 the surrounding undifferentiated cytoplasm. 



The nuclei gradually move inward as the cells assume their adult form 



