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WHEELER. [Vol. VIII. 



intercepting membrane between the nerve and the plate, whereas 

 the nerve passes without interruption into the ganghon. But I 

 am led to lay little stress on these appearances by the researches 

 of Watase ('90) and Parker ('90) on the adult ommatidia of a 

 number of Arthropods. They have shown in a very convinc- 

 ing manner that each retinula-cell is the termination of an 

 optic nerve fibre. The retinulae are undoubtedly modified optic 

 plate cells; and judging from recent observations on per- 

 cipient cells in other forms (vertebrate eye, ear, olfactory 

 nerves, v. Lenhossek's observations on Linnbricus {'92), etc.), 

 we must suppose that the nerve fibres grow out from the 

 retinula-cells, pass through the optic nerve and enter the 

 ganglion. Such prolongations from all the retinulae would be 

 amply sufficient to form the optic nerve, although it is probable 

 that some of its fibres are centrifugal prolongations from optic 

 ganglion cells. It has been suggested that the optic nerve 

 may be established at a very early stage, when the optic plate 

 and optic ganglion are still closely applied to each other (inde 

 Figs. 32-34), and that the nerve may not become visible till 

 the two Anlagen separate with further development. But I do 

 not think this is the case in Xiphidiiim. Sections like Fig. 

 36 show a distinct separation of the optic plate and ganglion 

 in the region of the future optic nerve; and Viallanes has made 

 an exactly similar observation on Mantis. 



The sympathetic nervous system arises in part at least from 

 the dorsal median wall of the oesophagus. At three separate 

 points (Fig. 61) the ectoderm becomes thickened and its outer 

 cells enlarge and assume the character of ganglion cells. The 

 most anterior of these thickenings (/. g) is the frontal gang- 

 lion. It arises just behind the base of the labrum. The two 

 other thickenings which are placed further back {I'g^, rg~) are 

 the second and third visceral ganglia. I have not followed the 

 development of the nerves which unite these ganglia and 

 ramify from them. 



Concerning the origin of the peripheral nervous system I 

 have no positive data. In a few cases I have seen appearances 

 which led me to believe that they arise as outgrowths from 

 their respective ganglia. 



