19 CENTRAL NERVOUS SYSTEM. 



turtle, is remarkable for the development of a beautiful group of cells, 

 situated in the median line, and first described by Stieda.* 



Plate LXXVIII. shows the same group in the Florida emys. It 

 is also present, but as a less striking feature, in saurians and ophidians. 



On either side of this central group, extending laterally and down- 

 wards around the margin of the ventricle, and in some cases parallel to 

 its epithelial lining, are successive layers of small nerve cells and nuclei. 



These cell strata are regarded as the origins of the fibres of the optic 

 nerves. In chelonians, some saurians and tailed batrachians, scattered 

 at unequal intervals among the cells, which form the strata, larger cells, 

 isolated or in scanty groups, are frequently found, as in Plate LXXXIV. 

 In the region beneath the optic lobes, called by Reissner the pars pe- 

 duncularis, are found the cells of origin and fibres of the oculomotorius. 

 Plates LXXVII. CX. & CXIII. The origin of the oculomotorius is 

 not demonstrable in tailed batrachians. 



Stieda compares the mid-brain of the axolotl with a tube, the open- 

 ing of which is spindle-shaped in cross sections, and expresses the opin- 

 ion that there is in reality but one lobus opticus, the common division 

 into pairs being based upon external appearances alone. f 



This view may be nearer the truth than the prevailing one, but can 

 hardly be reconciled with such appearances as those furnished by Plates 

 LXXI. & LXXII. In the siren and axolotl, it may be well to speak 

 of the lobus opticus ; but the plates referred to show, in the horned toad 

 and anolis, two distinct lobes, and what is more conclusive, in the plate 

 from the horned toad, commissural fibres are shown uniting these lobes. 

 This section, although cut in a vertical plane, is seen to include the optic 

 chiasm, and the cause is seen in Plate LXV. 



* Loc. cit, p. 63. 



t Axolotl, loc. cit., pp. 21-22. 



