320 S. WATASB. 



lose its force even if this section of my views in repjard to the 

 fate and liomoloiiy of the central cell or cells be jiroved unten- 

 able. It is quite possible that the oniniatidia in wliiidi there is 

 no element i-orrespondinj^ to the central cell of JAiivuIuh may 

 have originated without it from the beginning. It seems, how- 

 ever, more natural to suppose that such an ommatidium had it 

 originally and lost, it later, observing that the simplest form of 

 ommatidium possesses it in its fully functional, sensory form. 



Finally wc have to consider the nature of the compound eye 

 as a whole as presented in various types of Arthropods. 



That a certain structure in the body of an animal may repeat 

 itself and give rise to a secondary aggregate, or to a compound 

 organ, is a well known fact; the repetition of similarly con- 

 structed uriniferous tubules forms the essential part of a verte- 

 brate kidney, or the similar repetition of gill-filaments forms the 

 respiratory organ of a Lamellibranch. Sundry other examples 

 of this nature might be given, but the above two will suffice. 

 Tracing, as I have attempted to do, the most complicated omma- 

 tidium into a simple, open ectodermic pit, there is to my mind 

 no difficulty in believing that the compound eye of the Arthro- 

 pod is one of tiie most astonishing examples of the formation of 

 ail organ by the vegetative repetition of the similar structure. 

 Tlius, according to Lubbock, there are about 4000 facets in the 

 compound eye of the house-Hy {Musca), each facet corres- 

 ponding to a single tubular invagination of the skin, the omma- 

 tidium. There are 4000 independent invaginations in the area 

 in the head of the fly occupied by the com])ound eye; in the 

 gadfly {(.Edrus) 7000; in the goat-moth {Cossus) 11,000; in the 

 death's-head moth {Sjjhinx atropos) 12,000 ; in a butterfly 

 {Fajnllio) 17,000 ; in a dragon fly {^schna) 20,000 ; in a email 

 beetle {Mordella) as many as 25,000. On the other hand, the 

 number of ommatidia seems to have reached its minimum in 

 certain Copepods, as in Coi'ycceuK, where the whole visual organ 

 seems to be represented by a single colossal ommatidium. 



Certain forms of Collembola' seem to have a very small num- 



' Lubbock. Monograph of Ihe Collembola and Thysanura, the Ray Society, 

 1873, p. 57, Pis. LV and LVI. Lubbock uses the term " ocellus " to designate 

 a single element ot the eye which I here called an ommatidium. If the struc- 

 ture of this "ocellus" differs from the ommatidium of other Arthropods, it 

 has, of course, nothing to do with the discussion at issue. 



