Herrick, Morphology of Brain of Bony Fishes. 49 



region it could be readily proven that they have no relation to 

 the large, deeply staining and agglomerated fibres of the mesen- 

 cephalic root of V. P^ig. 9 illustrates one of many instances 

 where the cell lies entirely above the ental ganglion layer. It is 

 not at present possible to say whether sucii cells represent the 

 superficial layer of large ganglion cells characteristic of the tec- 

 tum of fishes. If so, it would seem natural to suggest a com- 

 mon origin of the two sets of tectal cells. 



The chief differences between the structure of the tectum 

 in fishes and reptiles seem to grow out of two elements. First, 

 the tectum is relatively much thicker in the latter and the ele- 

 ments are consequently much more crowded ; second, the layers 

 do not preserve the discrete character observed in the lower ani- 

 mals. There is a fusion of the several layers, especially the 

 layer of fusiform cells, which in fishes lies at a little distance 

 from the ventricle, is in Tropidonotus , for example, dispersed 

 among the other layers. The layer of large ganglion cells lying 

 beneath the superficial optic tract, is also dispersed throughout 

 the greater part of the thickness of the tectum. The above 

 changes necessitate a greater dispersion of the fibres so that the 

 latter no longer occupy distinct concentric bands or zones. 



Judging from the specimens examined, the young of reptilia 

 retain the concentric arrangement much more perfectly than the 

 adults. As for example the young Aspidonedes figured in Vol. I. 

 The following description of the tectum is given by Sanders. 1 



'' The structure of the tectum lobi optici differs m different 

 parts of its extent ; taken at about the centre of its arch, it shows 

 seven layers, commencing at the outside. The fiist layer con- 

 sists of finely granular neuroglia, placed in immediate contact 

 with the pia mater covering the tectum. The second layer con- 

 sists of coarse fibres, apparently transverse but really oblique, 

 that is, going in a direction between transverse and longitudinal; 

 this layer contains sparsely distributed fusiform cells, with their 

 long axes placed radially. The third layer consists of granular 

 matter, with closely packed fibres, arranged radially so as to 

 give it a smoothly striated appearance. The fourth layer has the 

 radial fibres not very closely placed, and the remainder of the 



1(3/. at. p. 75 1. 



