No. I.] EYES OF ARTHROPODS. 205 



Hickson the outer " Marklager" and that in the opticon, the 

 inner ^^ Marklager!' Viallanes (14) called the same substance, 

 according to its position, the " masse medullaire extcrne" " in- 

 terne," or " terminale." It does not seem advisable to adopt 

 the term " neurospongium," for the existence of any such struct- 

 ure has not been demonstrated, and if it had been, the supply 

 of terms already in use would be quite ample for its designation. 



Hickson is inclined to criticise (p. 228) Leydig's successors, 

 because they overlooked or misunderstood the nature of the 

 cells composing the " Punktsubstanz," which, he says, was de- 

 scribed by Leydig as a ner\'^e-cell sheath surrounding the other 

 parts of the ganglia. Leydig did not describe the " Punktsub- 

 stanz " as a sheath of nerve-cells, but, on the contrary, as a cen- 

 tral mass of net-like and interwoven nerve-fibres. 



Hickson takes some pains to confirm this supposed observa- 

 tion of Leydig, by showing that the Punktsiibstans in the silk- 

 worm moth is composed of nucleated cells. He probably had 

 in mind the '^ gratie granldre Rindensubstanz " which Leydig 

 showed to be composed of cells, or what he called ^^ ganglion- 

 kicgeln." 



The same author finds a variety of ganglion-cells in the optic 

 tract, among them some " apolar ones." He also affirms that gan- 

 glion-cells are seldom found in the optic tract of Insects, although 

 he says some are occasionally present. This is probably due 

 to the fact that, according to his definition, a ganglion-cell has 

 considerable cell protoplasm, and a nerve-cell comparatively 

 little. It is probable that there is a very similar difference be- 

 tween the "nerve-fibrils and fibrillae " (p. 227), which, accord- 

 ing to Hickson, form distinct constituents of the optic tract. 



According to Berger (p. 40), the layers of cells called a " ret- 

 ina" are to be regarded as such because they " stets innig mit 

 dem Licht percipirenden Apparat, den Sehstaben, verbunden 

 bleiben, wahrend die iibrigen Bestandtheile des Augenganglions 

 von demselben durch Nerven abgetrennt sein konnen." In the 

 majority of cases, however, the so-called " retina " of Berger is 

 separated from the ommateum by nerve-bundles more clearly 

 than it is from the optic ganglion. Moreover, the " retina " is 

 separated from the ommateum by the brain-sheath, which en- 

 closes the brain and all the parts of the optic ganglion and 

 *' retina " in one sac ; showing that the " retina " is far more 



