156 S. WATASE. 



tidium of the compound eye of an Arthropod is an independent 

 invagination of the skin. If this view is correct, the unit of 

 the compound eye of an Arthropod is not, after all, so com- 

 plex a structure as has been supposed by some; and the 

 enormous increase in the number of ommatidia in a given area 

 of the skin which results in the formation of the compound 

 finds its parallel in the well*known method of the formation 

 of the morphological organs, viz. the duplication of a simple 

 unit. 



DESCRIPTION OF PLATE XIX, 



Illustrating Mr. Watase^s paper on " The Morphology of the 

 Compound Eyes of Arthropods.^^ 



Figs, 1 — 5. — Diagrams showing the probable evolution of the three- 

 layered ommatidium from the single-layered surface depression in the skin 

 by the gradual subsidence of the neuro-epithelial elements, Rt. and G., Fig. 

 1. In Fig. 2 the ommatidium of Limulus is represented, which is con- 

 sidered a step further advanced from the condition shown in Fig. 1. The 

 distal end of the retinula {Rt.) instead of being pointed toward the exterior 

 as in Fig. 1, in Limulus points towards the median axis of the omma- 

 tidium. The chitiuous substance being still secreted on the outside, a distinct 

 body of chitin beneath the lens-cone (C.) is formed, the rhabdom {Rb.). In 

 Fig. 3 this deepening is supposed to have gone still further, resulting in the 

 formation of another independent chitinous body, the crystalline cone {c.c). 

 In Fig. 4 this deepening is considered to have advanced still further, the 

 crystalline cone {c. c.) being entirely separated from the corneal lens {C) by a 

 distinct stratum of cell, the corneagen {eg.). In Fig. 5 an ommatidium Avith 

 three strata of cells, each secreting chitinous substance on the part of their 

 surface, is formed. These three strata of cells are known as the corneagen 

 ((V7.), the vitrella (F.), and the retinula (/2if.). Three chitinous bodies 

 secreted by each group of cells above mentioned are the cornea (C), the 

 crystalline cone (c. c), and the r habdom ere (iJi.), respectively. 



Fig. 6. — Serolis. Diagram of the ommatidium of Serolis. The general 

 arrangement of cells in this is not very different from that shown in Fig. 5 

 The place of ganglion-cell in Fig. 5, G., is taken by a pair (of which only 

 one is shown in the diagram) of transparent " hyaline cells " (//.). 



