408 
PEOE. B. T. LOWNE ON THE 
Weismann’s ‘ Stiel ’ was the optic nerve, and his 1 Augenscheibe ’ 
the structure from which the dioptron is developed. I shall have 
later to give Dr. Weismann’s views more fully. Dr. Hickson 
continues (page 27) : — “ Since Berger’s paper appeared Carriere 
has described the periopticon as ‘ a layer of long palisade-sbaped 
cells, the number of which corresponds with the eye units; every 
one of these palisade cells possesses an oblong nucleus at its 
foremost, somewhat broader, end.’ My researches show that 
this description is quite inaccurate. The elements of the periop- 
ticon are not cells, and the large oval nucleus situated in each 
element does not exist ; nerve-cells, when they exist in the region 
of the periopticon in Muscct, lie between the elements and not in 
them, as my figures show.” 
These statements and others show that Dr. Hickson and Carriere 
do not agree. With regard to the nuclei described by Carriere, 
they undoubtedly exist, but not, as Carriere thought, within the 
palisades, but externally to them, immediately beneath their in- 
vesting sheath. Dr. Hickson is right when he says these bodies 
are not cells, they are developed from cells, and each consists of a 
bundle of fusiform rods. With regard to the terminations of the 
optic nerve, Carriere distinctly traced the nerve-fibres into the pali- 
sades ; Dr. Hickson says they go round them. I trace them directly 
into the fusiform rods which form the palisades. The structures 
seen and correctly figured by Dr. Hickson are tracheal vessels. 
Carriere supposed the nerve-fibres to pass out at the superficial 
end of the palisades and to perforate the basilar membrane ; from 
this I entirely dissent. In support of this view Carriere has 
figured, quite diagrammatically, what I believe is a tracheal vessel 
seen behind the fusiform body. Carriere also saw the highly re- 
fractive outer ends of the rods, or, rather, that part which is con- 
nected with their inner portion, and says, “ in JSLusca vomitoria 
one sees in every cell a cylindrical axis.” 
Dr. Hickson entirely put himself in the wrong in describing 
the nervous elements as between the palisades ; bis nervous 
elements are undoubtedly fine tracheal tubes. Dr. Hickson’s 
figures accurately represent the nerve-sheaths and tracheae as 
well as the supporting neuroglia, but no vestige of nerve or 
nerve-end organs appears in them. A careful examination of his 
own figures at once leads to a dissent from all his statements, 
which are as inaccurate as his figures are accurate. I cannot 
understand how so good an observer could have been so misled. 
