514 THI-: SENSES AM) SI:\'SORY ORGANS. 



The Development of the Simple Eyes. Unfortunately, the 

 observer is met by great difficulties in the attempt to settle the 

 question at issue by the study of the development of the simple 

 eye. The earliest stage in which I have been able to identify 

 these organs is the third or fourth day of the pupa, when the 

 nerve is easily traced from the nerve centre to the integument. 

 In this stage the nerve ends in three clavate groups of pyramidal 

 cells, with their narrow ends towards the nerve, and their broad 

 ends applied to the undifferentiated hypoderm beneath the thin 

 cuticle of the vertex. There are no traces of either vitreous or 

 lens at this period, and I can only regard the clavate groups of 

 cells as rudimentary retinae developed in relation with the 

 nerve. There is apparently no trace of vesicular structure, and 

 there is certainly no involution of the undifferentiated hypo- 

 derm, the cells of which subsequently form the vitreous and 

 lens. The appearances presented at subsequent stages indicate 

 that both these structures are developed from the cells of the 

 hypoderm by the growth and differentiation of their substance. 

 There is never the slightest indication of involution or invagi- 

 nation. 



Although, with the exception of those of Mark [241] and 

 Locy [240], no direct observations as to the manner in which 

 the median stemmata of Insects are developed have been pub- 

 lished, Leydig [207, PI. VI., Fig. 5] gives a figure of the simple 

 eyes of Timarchia tenebricosa, in \\rfiich they are represented 

 in the imago as vesicular organs unconnected with the integu- 

 ment, but directly attached to the optic ganglia. This figure is, 

 in my opinion, only susceptible of one interpretation: that the 

 retina is produced as an outgrowth from the central nervous 

 system. 



The Ocelli compared with the Pineal Eye- This comparison is 

 suggested by Gaskell [45], and there are certainly several very 

 Striking points of Similarity. The pineal eye is undoubtedly a 

 rudimentary otgan, which tends to disappear even in the lower 

 Vertebrates; whilst the ocelli are characteristic of the lower 

 Arthropods, and tend to be rep]. iced by th" compound eyes, 

 functionally at least, in all the higher forms. In both cases the 



