Eyes of Molluscs and Arthropods. 599 



be seeii. at tlie base of tlie Ophthalmie fold, bctweeu it aiul the velimi, 

 a few large cells coustituting small ovai thickeuiugs, the rudiments of a 

 future eye (fig. 18)^ 



The cells of the thickeuing are large and coluinnar, striatcd at the 

 outer extremìties, while at the inner end they are clearer and bounded 

 by faint lines. lustead of tcrmiuating in a sharply defìned boundary, the 

 protoplasm is often drawn out into mauy amoeboid-like arms, in the 

 larger of which are nuclei. The boundaries between the cells are very 

 faint and often invisible. The cilia, which are usually confiued to the 

 apical lobes (fig. 2), in this case extend over the optic thickening 

 (fig. 18). On the outer edge of the rudimentary eye (that is away 

 from the hingeì , the nuclei are smaller and show a tendency to form a 

 double row. In the next stage (fig. 14), au increased development is indi- 

 cated by the greater number of nuclei on the outer side of the thickening, 

 and their tendency to form several rows. They are likewise smaller than 

 those formìng a single layer üu the opposite side. In this example also, 

 the cilia have extended over the optic thickening, but they are fewer 

 and smaller and appear to be degeneratiug. Both figs. 14 and 18 are 

 sectious of larvae, 1 mm long, in which no pigment was developed. 

 In these young specimens, the absence of anythiug like a basai 

 membrane, and the manner in which the inner ends of the cells are 

 drawn out into naked, protoplasmic, amoeboid arms, some of which 

 contain nuclei, is worthy of notice. 



By continued proliferation of the cells on the outer side of the 

 optic thickening, an ovai, knob-like papilla is formed, the long diameter 

 of which is parallel with the optic tract (fig. 11). 



The optic tract is now pigmeuted, as well as the base of the pa- 

 pillae at whose summit (fig. 11) the pigment is absent. The wall of the 

 thickening becomes better defined and consists of a single row of regu- 

 larly arranged nuclei, except on the outer side where the proliferation 

 to form the hypodermic core continues, thus, at thatpoint, prevent- 

 iug the formation of any sharp line of demarkation between the super- 

 ficial hypodermis and the underlying cells. The diifereuce in the 

 manner of division between the cells of the upper and lower side of the 

 optic papilla gives rise to a change in form and direction of growth. 

 At the point ìiaj. fig. 1 1 , the piane of division is parallel to a tangent 

 at the surface, andtherefore the cells grow inwards to form the core. 



' For the sake of conveniecce in the following description, we shall speak of 

 the pigmented band, from which ali the eyes originate, as the «optic tract«. 



