46 BULLETIN OF THE 



area. Of course the number of pairs of hypodermal nuclei equals the 

 number of ommatidia. In the earliest stages in which the ommatidia 

 have been seen there were always three or four pairs of hypodermal 

 nuclei, so that it is probable that the first step in differentiation is the 

 simultaneous production of three or four ommatidia. 



The second kind of nuclei in the superficial layer are roundish and 

 arranged in circles of six around each pair of hypodermal nuclei. The 

 circles are so combined that each nucleus plays a part in three circles ; 

 and as there is one circle for each ommatidium, it follows that only one 

 third of each nucleus belongs to a given ommatidium, or, if one esti- 

 mates the nuclei as units, only one third of each circle of six nuclei, or 

 two nuclei, belong to an ommatidium. These nuclei represent the cells 

 which in the adult have been called the distal retinulse, and of which 

 there was a single pair to each ommatidium. 



The deep layer in the distal band of nuclei lies directly below the 

 superficial layer (Fig. 42). The nuclei which constitute this layer are 

 all of one kind, and are arranged in groups of four (Fig. 44). They 

 represent the cells of the crystalline cones. The centre of a given group 

 of cone-nuclei is directly below the centre of a pair of hypodermal nuclei. 

 At this stage the cone-cells can be observed as elongated pyramids, 

 which lie with their bases in the region of their four nuclei, and their 

 apices extending into the deeper part of the retina (Fig. 42). 



The nuclei of the proximal band show at this stage no special arrange- 

 ment. They migrate to the deeper part of the retina, and there undergo 

 further change. Between them and the basement membrane the pig- 

 ment is deposited. 



The most noticeable changes which the retina as a whole now under- 

 goes are two. First, it thickens until it is throughout nearly as thick 

 as in the region where the ommatidia were first differentiated. (Com- 

 pare Figs. 45 and 46.) Second, the number of ommatidia greatly in- 

 creases. These two changes, the thickening of the retina and the 

 production of new ommatidia, go hand in hand and spread over the 

 general surface of the eye, from the region in which the first ommatidia 

 appear. It is worthy of notice that the new ommatidia are constructed 

 from the undifferentiated cells which immediately surround the area of 

 ommatidia already formed. Cells once incorporated in a given omma- 

 tidium never in any way contribute to the formation of other ommatidia. 

 Moreover, in the differentiation of an ommatidium no cells are left 

 between it and the neighboring ommatidia, so that ommatidia once ad- 

 joining each other remain so. In other words, new ommatidia are not 



