514 THE SENSES AND SENSORY 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 retin 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 
I_ocy [240], no direct observations as to the manner in which 
the median stemmata of Insects are developed have been pub- 
lished, Leydig [207, Pl. VI., Fig. 5] gives a figure of the simple 
eyes of Timarchia tenebricosa, in which they are represented 
in the imago as vesicular organs uncennected 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 organ, which tends to disappear even in the lower 
Vertebrates ; whilst the ocelli are characteristic of the lower 
Arthropods, and tend to be replaced by the compound eyes, 
functionally at least, in all the higher forms. In both cases the 
