No. r.] DEVELOPMENT OF THE COMPOUND EYE. 55 



been a development and a differentiation. Each of the cells 

 (Fig. 10) has become greatly elongated, the protoplasm ex- 

 tending out to a considerable distance from the nucleus in a 

 thread-like prolongation, which apparently (Fig.ll, 13) reach 

 from the wall of the optic cavity to the epidermis. The 

 nuclei are placed at different heights in these cells, and 

 the tail-like prolongations are arranged in layers around the 

 crystalline cone. Exactly which of the cells are inside and 

 which outside I am not able to determine, except in the case 

 of the distal member of the retinal row (r). This is clearly 

 the one which abuts against the crystalline cone, and is 

 the crystalline cone-ce41 or retinophora. Of these there are 

 four surrounding the cone, and their walls touch so that they 

 form a cup or calyx in which the cone is situated, and from 

 which it is secreted. Below the calyx the ends of the retino- 

 phoral cells unite to form a slender pedicle (ypd), which traverses 

 the whole of the pigmented layer as it at present exists. This 

 pedicle is clearly the rhabdom of Grenacher, and it is as plainly 

 formed, as was first demonstrated by Patten, by the retinophorae, 

 and not as a secretion from the surrounding pigment-cells. On 

 this point I am fully convinced of Patten's accuracy. 



The retinophoral nuclei still retain their position (as they do 

 throughout life) close to the epidermis, but among the others of 

 the retinal rows a separation is visible. Of these cells, which 

 we may call the pigment-cells, two remain in close contact with 

 the retinophoral nuclei, {p^, p^, Fig. ii) while the two re- 

 maining nuclei {p^, /^) are separated from the rest and from 

 each other by a considerable interval. A comparison of Figs, 

 8 and 11 will show what has taken place. Pigment-cell 4 has 

 retained its position at the margin of the optic cavity, but be- 

 tween it and pigment-cell 2 a considerable interstitial growth has 

 occurred, mostly accomplished by an elongation of the proto- 

 plasm of the cell. The deposition of pigment, which, in Fig. 

 8, had just begun at the proximal surface has now covered the 

 entire nucleus, and has extended thence up the filiform pro- 

 longation of the cell (see also Fig. 10) until it has reached the 

 level of the nucleus of pigment-cell 2. These prolongations 

 are the well-known rods or bacilli {b) so characteristic of the 

 arthropod eye. The mesoderm still retains its primitive charac- 

 ter at this stage, and as yet no connection is made between the 



