156 THE EYE IN EVOLUTION 



eye ; dim. ofxixxTlSiov) ; the typical formation of the whole eye is 

 seen in Figs. 134 and 150. 



The developnietit of ocelli and cotnpound eyes indicates their essential kinship 

 despite their outward disjDarity of form. The oceUus, as we have seen, originates 

 as a hypodermal pit, the superficial cells of which, infolding under the cuticle, 

 become differentiated into a refringent apparatus, the deeper cells into the 

 retinal elements. Each ommatidium of the compound eye originates some- 

 what similarly as a consolidated pillar of hypodermal cells and between the 

 pillars lie undifferentiated cells (Fig. 135) ; the superficial cells of these pillars 

 form the basis of the corneal facets, the crystalline cones and primary pigmented 

 cells, the deeper cells develojD into the retinviles, while those between the pillars 

 form the secondary pigmented cells. In both cases the baseinent membrane is 

 continuous with that of the integument (Patten, 1888-1912 ; Johansen, 1893 ; 



A 





Fig. 135. — The Development of the Compound Eye. 



An early stage in the development of the eye of the pupa of the moth, 

 Saturnia pernyi, showing the ommatidial pillars (after Bugnion and Popoff ). 



Bugnion and Popoff, 1914). It wovild thus seem that ontogenetically as well as 

 phylogenetically the two types of eye are parallel developments from some 

 (unknown) common primitive origin. 



While ocelh and compound eyes show this kinship in development, the studies 

 of Watase (1890) and Hanstrom (1926) would indicate that they have a different 

 origin ; all true compound eyes arise from the lateral ectodermal mass in the 

 embryo, while ocelli take origin from either the dorsal or the ventral ectodermal 

 mass. Although the lateral ocelli of modern arachnids and all the eyes of 

 diplopods and chilopods arise from the lateral mass, Hanstrom considers them 

 to represent degenerate forms of the ommatidia of compound eyes. 



It woiild thus seem reasonable to assume that the compound eye has evolved 

 from the simple eye at an early period, but it is clear that the first is not an 

 adaptive modification of the second after it has reached an elaborate stage of 

 development. It is true that intermediate stages are extant — the association 

 of the sensory cells into a group under a single common lens, seen in the simple 

 ommatidial e of some larval and adult insects and Copepods (Fig. 138), or the 

 multitubui. rangement of the aggregate eye wherein each element contains a 

 single senses -U, seen in some polychsete worms (Fig. 128). It is significant. 



