1905.] NATURAL SCIENCES OF PHILADELPHIA. 155 



which later become arranged as a spindle surrounded by smaller cells 

 (p. 136). 



This spindle is the retinula, and the cone cells and pigment cells 

 assume a distal position by a morphological imagination (p. 137). 



The retinula is the centre of the ommatidium, and the cone cells, 

 corneal pigment cells and outer pigment cells follow in the order 

 named (p. 141). 



The ommatidium is composed of eight or nine retinula cells around 

 the rhabdome, four cone cells, two corneal pigment cells and about 

 twelve outer pigment cells (p. 126). 



The rhabdome and cone arc intracellular secretions, while the lens 

 is an extracellular secretion of the pigment cells (p. 144). 



The corneal pigment cells are homologous with the corneal hypoder- 

 mal cells of crustacean and apterygote insect ommatidia (p. 147). 



The innervation of the ommatidium is by a differentiation of part of 

 the retinular cells into nerve fibrils, and these extend to the retinular 

 ganglia (p. 149). 



The lens is secreted by the corneal pigment cells which early in the 

 pupa stage lie distal to the cone, and possibly also by the outer pigment 

 cells (p. 148). 



Pigment is formed inside all the cells of the ommatidium, except the 

 cone cells, by a cytoplasmic differentiation (p. 149). 



The ommatidium arises from a strictly one-layered epidermis, which 

 passes directly from the larva to the pupa without the loss of any cells 

 or additions from other tissues (p. 136). 



LITERATURE. 



BALFOUR, F. M. 1881. A Treatise on Comparative Embryology, II, London. 

 BUTSCHLI, O. 1870. Zur Entwicklungsgeschichte der Biene. Zeit. /. Wiss. 



Zool XX, pp. 519-564. 

 CARRIERS, J. 1884. On the Eyes of Some Invertebrates. Qt. Jr. Micr. Sc., 



XXIV, New Series, pp. 673-81. 

 1885. Die Sehorgane der Thiere vergleichend anatomisch dargestellt. 



Mtinchen und Leipzig, R. Oldenbourg, 6 + 205 pp. 



1885. Einiges uber die Sehorgane von Arthropoden. Biol. Centralbl., 



V, No. 19, pp. 589 T 97. 



1886. Kurze Mittheilungen aus fortgesetzten Untersuchungen viber die 



Sehorgane. Zool. Am., IX, Nos. 217 and 230. 



FERNALD, H. T. 1890. The Relationships of Arthropods. Studies Biol. Lab. 



Johns Hopkins Univ., IV, pp. 431-513. 

 GRENACHER, H. 1874. Zur Morphologic und Physiologic des facettirten Ar- 



thropodenauges. Gottingen Nachrichten, pp. 645-56. 

 1877. Untersuchungen iiber das Arthropoden-Auge. Beilageheft zu 



den kHnischen Monatsbldttern fur Augenheilkunde, XV. 



1889. Untersuchungen iiber das Sehorgan der Arthropoden, insbesonder 



des Spinnen, Insecten und Crustaceen. Gottingen, Vanderhoeck und 

 Ruprecht, 8 + 188 pp., 11 Taf. 



