408 



PROF. B. T. LOWNE ON THE 



Weismann's ' Stiel ' was the optic nerve, and his 'Augenscheibe ' 

 the structure from which the dioptron is developed. I shall have 

 later to give Dr. Weismaim'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-shaped 

 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 Musca, 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 riglit 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 diagram matically, 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 Musca 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 ; his nervous 

 elements are undoubtedly fine tracheal tubes. Dr. Hickson's 

 figures accurately represent the nerve-sheaths and trachese 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. 



