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SYDNEY J. HICKSON. 
Carriere (3) says that only five nuclei lie in the uppermost 
end of the retinulse. My own researches lead me to believe 
that six nuclei are found in the uppermost part which belong 
to the six retinulse, and that the one or two nuclei which are 
found about half way down are additional nuclei belonging to 
one or two of these six retinulse. In other words, 1 believe 
that some of the retinulse possess two nuclei, and my opinion 
upon this point is supported by the investigation of the 
same structures in other Insects and Crustacea, and by 
the researches of Claparede and Weissman upon their de- 
velopment. I cannot see anything morphologically monstrous 
in supposing that the retinula cells possess more than one 
nucleus. 
The inter-ommatidial tracheal vesicles have been known to 
exist for some time. They were mentioned by Swammerdam, 
who said : “ Es ist wunderbar, wie und in was fur einer grosseu 
Menge die Luftrohren .... hinaufklimmen,” by Serres, by 
Will, and by Leydig (11, 12). Grenacher (7) figures a 
portion of one of them in Tabanus bovinus (Taf. vii, 
fig. 60), and they are mentioned by Lowne. By some autho- 
rities they have been supposed to perform the function of a 
tapetum owing to their brilliant glittering appearance in the 
living eye, but the sheath of pigment-cells surrounding the 
ommatidia, and between them and the tracheal vesicles, mili- 
tates against this view ; and they must be regarded, I think, 
simply as vessels for the aeration of the ommateal tissues. 
They were also known to Ciaccio (4), whose description of the 
distribution of the tracheae to the optic tract in the Diptera is 
exceedingly accurate. Berger (2) also mentions the tracheal 
diverticula, but does not figure them. 
The function of the retinulse has been a subject which has 
been very largely discussed by the older writers, but the balance 
of opinion has always been in favour of considering the 
retinulse to be the true nerve-end cells. This was most cer- 
tainly the opinion of Muller (17), but although this was 
combatted by Gottsche (6), and by Wagner (21), who thought 
he could trace the nerve-fibrils through the retinulse to the 
