COMPOrXD EYt:S OF .t/rr/fROPOPS. :529 



is round, consisting of two parts, Rl (retinulae) and G (central gang- 

 lion cell). Cells forming the walls of the pit are the perineural 

 cells (;>.«). Inward prolongations of the neuro-epithelial cells 

 (Rt and (?) form the optic fibres {Op. n). Ms. JLesodermic tissue. 



Fig. 6(1. — Limulus. Transverse sections of five oniniatidia at 

 different levels. In («) the plane of section passes through the 

 upjier part of the depression, cutting the lens-cone (L); in (6) the 

 section plane passes through the retinulae, showing the star-.shaped 

 rhabdom and the axial process of tlie ganglion cell (Ax.p) in the 

 axial canal of the rhabdom. A few pigtnented perineural cells sur- 

 round the retinulae, while the basement membrane (B. M) forms 

 the complete capsule of the ommatidium, separating the latter from 

 the mesodermic tissue {3h). In (c) the plane of section passes 

 through the lower part of the ommatidium, cutting the central 

 ganglion cell {G); in (d) the ganglion cell occupies an eccentric 

 position ; in (e) the sections of optic nerve fibres alone are seen. 



Fig. 7.— Limulus. Plan of the ommatidium. Ep represent the 

 cells which are found in the walls of the pit; the yellow edge 

 corresponds to the surface where the chitiu is secreted. 



Plate XXX. — Limulus. 



Fig. 10. Retinula (7?/) and ganglion cell ((?) teased out from 

 the ommatidium after treatment with Ilaller's fluid 15 hours. The 

 axial process (Ax.p) of the ganglion cell closely follows the external 

 border of the rhabdomere (Rb). A number of epithelial cells are 

 found closely attached to tiie surface of the retinula as well as to the 

 ganglion cell. 3 (E X 2) Z. 



Fig. 11. A bunch of retinulae completely isolated from the 

 ommatidium. Pigmented epithelial cells (Ep) are found between 

 them. The proximal ends of retinulae form the optic nerve fibres 

 (Op.n). J(EX2)Z. 



Fig. 12. A retinula cell seen from the side opposite to the rhabdo- 

 mere. A number of extremely attenuated epithelial cells (Ep) are 

 found closely clinging to the external surface of the cell. They do 

 not form any connection with the retinula, and can be completely 

 isolated from it. The cell-body of such an elongated epithelium is 

 reduced to a very slender filament and does not contain any pigment 

 granules. 5 (E X 2) Z. 



Fig. 13. The central ganglion cell with its axial process (J. t./;) 

 completely isolated. The pigment patch {P(j.p), situated between 



