294 .s'. WATASE. 



arc tlio (inly cells whirli arc connected with tlic nerve centre. 

 In iidJition to the tlireo elements above naniei], tlie fourtli 

 element, in the form of two " iiyaline cells," exists, plugging, as it 

 were, from the inside the imperfect bottom of the depression. 

 A number of pigment cells enveloping more or less completely 

 the above mentioned group of cells from the outside constitutes 

 the fifth element of the ommatidium. 



A glance at the diagram (Fig. lb, PI. XXIX) will make 

 this point clear. The diagram is supposed to be the plan of the 

 constructive elements of the ommatidium of Serolis as they 

 would appear when seeii from the exterior, in the direction of 

 the arrow. Fig. la, PL XXIX, if they actually foruicd an 

 open pit like that shown in Fig. 2. 



The innermost body which appears in the depression is the pair 

 of "hyaline cells," {d)\ next above and therefore in the outer 

 circle to {(I) are.the retinulae (c), the rhabdomeres being repre- 

 sented by the yellow edges. Above the retinulae will come the 

 two extremely enlarged cells, the vitrellae {b), whose chitin- 

 secreting surface is marked by the yellow color. Next above {b) 

 comes a pair of corneagen cells {a), whose chitin-secreting surface 

 is also represented by the yellow color as in the cells forming the 

 two inner series. The outermost of all is a pair of extremely 

 flattened, pigmented cells, ensheathing completely the dioptric 

 portion of the ommatidium. 



The diagram is not unlike the one used in illustrating the 

 arrangement of the difierent parts of a flower. This does not 

 seem surprising when we remember that the nature of the prob- 

 lem in both cases is identical. Just as sepals, petals, stamens and 

 pistils of a flower are considered as the modified leaves clustered 

 round an extremely short stem or axis in whorls or in spirals, so 

 the corneagen, vitrellae, retinulae, etc., are the modified ectoderm 

 cells arranged at difierent heights on the ommatidial axis. There 

 is, however, this diflerencc between them, that while in a flower 

 the parts concerned in the formation of the organ are themselves 

 aggregations of many cells, those in the Arthropod ommatidium 

 are highly difierentiated individual cells; and while in the case 

 of the flower it is the clustering of difierent parts around an 

 extremely shortened projecting axis, in the ommatidium of the 

 Arthropod it is the distribution of parts along an elongated and 

 sunken one. 



